At an annual address on the state of homeland security, Nielsen also pointed to homegrown terrorism and state-sponsored hackers. She said her agency is working with the State Department to put countries on notice when they don’t meet new "more stringent" requirements "to crack down on terrorist travel."
"Governments who work with us will make the world safer from extremists, while those that fail to comply will face consequences," she said.
The remarks come after a self-described white supremacist allegedly killed 50 people at two mosques in New Zealand.
"We will not permit such hate in our homeland," Nielsen said Monday.
Nielsen avoided discussing specifics of the alleged attacker's motivation and didn't mention the suspected shooter's beliefs. She instead focused on cases of Islamic State-inspired plots in the U.S.
Democratic Rep. Bennie G. Thompson, chairman of the House Homeland Security Committee, criticized Nielsen for not prioritizing the "very real threat of white nationalism and supremacism."
"It’s clear that she still does not have her priorities straight," Thompson said.
Over the weekend, a former White House national security adviser told ABC News' chief news anchor George Stephanopoulos on "This Week" that the Trump administration is largely prepared to invest more in curbing the domestic terror threat.
"I think that this administration stands ready to invest more money," said former Trump adviser Tom Bossert, who is now an ABC News contributor.
On Monday, Nielsen highlighted a domestic security case involving a Georgia woman accused of working for a group of ISIS hackers. The woman was arrested last week for allegedly creating ISIS "kill lists" of potential targets and serving as a recruiter for the terror organization.
Nielsen said that identifying ISIS followers remains a priority, despite the department's success in stopping foreign-born attackers from coming to the U.S.
"It is not an accident that since 9/11 there has not been a successful attack on U.S. soil by a foreign terrorist," said John Cohen, a former DHS senior official and ABC News contributor.
Nielsen also called out Russia for interfering in the 2016 election and said the country leads online influence campaigns that continue to pose a threat. The remarks came in contrast to President Donald Trump's previous dismissals of U.S. intelligence reports connecting the Russian government to election interference.
"They have continued to interfere in our public affairs and have attempted to sow division online among Americans on hot-button issues," Nielsen said.