Biden took aim at Trump in remarks following a briefing on national security and foreign policy, slamming the current administration for failing to fully cooperate with his agency review teams during the transition -- which Biden argued could leave the country vulnerable.

While commending his own agency review teams for their hard work amid a transition that was already made more challenging by COVID-19, Biden pointedly called out Department of Defense and White House Office of Budget and Management political appointees for, he said, not cooperating for a smooth transition.

"And the truth is many of the agencies that are critical to our security have incurred enormous damage. Many of them have been hollowed out in personnel, capacity and in morale," Biden added.

Later in his remarks, Biden stressed the need for cooperation during the transition, noting that it is a matter of national security to ensure there are no gaps that adversaries can seek to exploit when he takes office.

"We need full visibility into the budget planning under way at the Defense Department and other agencies in order to avoid any window of confusion or catch-up that our adversaries may try to exploit," Biden said. "Right now, we just aren't getting all the information that we need from the outgoing administration in key national security areas. It's nothing short, in my view, of irresponsibility."

Biden said other "Day One" challenges discussed in the briefing will draw on the skill sets of the Department of Homeland Security and the Federal Emergency Management Agency as they aim to "clean up the humanitarian disaster that the Trump administration has systematically created on our Southern border."

"We will institute humane and orderly responses. That means rebuilding the capacity we need to safely and quickly process asylum seekers, without creating near-term crises in the midst of this deadly pandemic," Biden said.

As he exited the stage at the Queen Theater in Wilmington, Delaware, Biden was asked if he supports the measure for $2,000 direct payments to Americans for COVID-19 relief, which the House will soon vote on. He responded with one word: "Yes."

-ABC News' Molly Nagle, Sarah Kolinovsky and Beatrice Peterson

In her first meeting as interior secretary-designee, Deb Haaland hosted a virtual roundtable with tribal leaders on Monday morning, according to the Biden transition team.

Joined by incoming senior adviser to the president and director of the White House Office of Public Engagement Cedric Richmond and incoming director of the White House Office of Intergovernmental Affairs Julie Rodriguez, Haaland “reiterated President-elect Joe Biden’s promise to Tribal Nations and indigenous communities to fully honor America’s sacred trust and treaty obligations to Tribal Nations,” the transition said in a readout of the meeting.

“They also discussed how the incoming Biden-Harris Administration will work in coordination with tribal communities who bear disproportionate harm from long-running environmental injustices and are being adversely affected by the impacts of climate change on their homelands,” the readout continued.

Haaland addressed Biden’s ambitious climate policy during the meeting, discussing the incoming administrations plans to address the inequities of global warming and efforts to address climate change, treating it as an economic opportunity. She also pledged to ensure that the Interior Department engages in "early, frequent, and meaningful consultation" with tribal leaders on decisions that affect indigenous communities, according to the readout.

If the Senate confirms her as secretary of the interior, Haaland would be the first Native American to serve in a presidential Cabinet and would be the first Native person to oversee an agency that's played a major role historically in the forced relocation and oppression of Indigenous people.

-ABC News' Molly Nagle

Roger Stone told ABC News he saw Trump "in passing" Sunday night at the Trump International Golf Club in West Palm Beach, Florida, and thanked the president in person for pardoning him last week.

"My wife and I both had the opportunity to thank the president personally for righting the injustice of my conviction in a soviet-style show trial, which featured the epic bias of the judge who withheld exculpatory evidence from my defense, misconduct by the jury forewoman and substantial misconduct by the prosecutors," Stone told ABC News Monday.

"Donald Trump is the greatest president since Abraham Lincoln," he added.

Stone told ABC News he and his wife, Nydia Stone, were the guests of Newsmax publisher Christopher Ruddy for the club's buffet dinner, where they ran into Trump, who was dining with family.

"The president decided to dine at the club with his son Donald Trump Jr., Kimberley Guilfoil, Eric Trump and his wife Lara Trump. Because of his decision to sign the stimulus bill, it was unclear whether the president would dine at the club, which he normally does on Sunday nights when he is visiting Mar-A-Lago," Stone said.

The White House declined to comment to ABC News on the matter.

Because the club rules prohibit photographs of diners for privacy reasons, the Stones declined to provide a photo. However, ABC News reached a source who was present in a dining room of about 100 diners who provided a photo of Trump of greeting Stone with a pat on the shoulder.

Stone, a decades-long friend and former campaign adviser to Trump, was convicted on a seven-count indictment of obstructing justice, witness tampering and multiple counts of lying to Congress in special counsel Robert Mueller's investigation into Russian meddling in the 2016 presidential election in November 2019.

In July, days before the veteran GOP operative was scheduled to report to a federal penitentiary in Georgia, Trump commuted Stone's 40-month prison sentence.

Trump's full presidential pardon, issued Dec. 23, nullifies Stone's conviction entirely.

-ABC News' Ali Dukakis

Biden and Vice President-elect Kamala Harris are participating in a virtual briefing with expert members of their national security and foreign policy agency review teams from the Queen Theater in Wilmington, Delaware, and taking the opportunity to draw stark contrasts between Trump's "America First" policy and their incoming White House.

At the top of the briefing, Biden praised the team for their work despite delays in the transition prompted by the Trump administration refusing to recognize Biden's ascertainment as the president-elect for more than two weeks.

"You’ve done it all under incredibly difficult circumstances, dealing with the COVID protocols, delays, delays in the ascertainments, and in a few cases obstruction from the current leadership," Biden said, thanking the group before taking aim at Trump's "go-it-alone approach" of the past four years.

“The truth is, the challenges we face today can't be solved by any one country acting alone. They demand American leadership. They demand cooperation with our allies and our partners. So I’m looking forward to hearing your assessments about the greatest challenges we may inherit and as we restore the principle of American leadership on the global stage -- which we have to and will do," Biden added.

After the briefing, Biden is slated to deliver afternoon remarks on the group's findings.

-ABC News' Molly Nagle

Following a briefing with his COVID-19 advisory team on Tuesday, Biden is expected to continue his criticism of the Trump administration in afternoon remarks on the coronavirus pandemic from Wilmington, Delaware, and home in on what his team has characterized as the lagging pace of vaccine distribution.

According to a transition official, Biden will provide an update on the current state of the pandemic and give a dire prediction that, tragically, infection and death rates will rise in the coming weeks. Biden is also expected to address the current administration falling short on its pace for vaccinations, the official said, as well as his plan to get 100 million vaccinations in 100 days.

Biden on Monday slammed the Trump administration for, he said, leaving security agencies "hollowed out" and for creating "roadblocks" for his transition team seeking access to key national security information -- a comment that acting Defense Secretary Christopher Miller has pushed back on, saying the Biden team has already been given more interviews and documents than they initially requested.

Vice President-elect Kamala Harris and her husband, Doug Emhoff, on Tuesday morning are slated to receive their first doses of the COVID-19 vaccine in Washington, D.C., where Harris may also comment on the incoming administration's vaccine distribution plans.

Trump, meanwhile, continued to contest the election on Twitter Tuesday morning -- with three weeks until Biden's inauguration -- criticizing Republican leadership as "pathetic," saying they "only want the path of least resistance" and "only know how to lose." He also attacked Republican lawmakers over their handling of the defensing funding bill as the Senate is expected to override Trump's presidential veto for the first time in his presidency by the end of the week.

After Trump changed course on Sunday and signed the coronavirus relief and government funding bill to avert a shutdown -- though not in time to prevent unemployment benefits to millions of Americans from lapsing -- the House of Representatives on Monday approved Trump's call to increase the coronavirus relief payments from $600 to $2,000.

But its fate in the Senate is uncertain. Senate Majority Leader McConnell has declined to reveal how he will proceed or even whether he’ll bring the proposal up for a vote. Democrats are seizing the opportunity to force Republicans into a difficult position -- either backing or defying Trump.

-ABC News' Molly Nagle and Ben Gittleson