Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, spoke with his military counterparts in France, Poland, Romania and Bulgaria Thursday about the latest developments in Ukraine, according to the Pentagon.

While the Pentagon didn't provide many details about his discussions with the military leaders, it said they shared assessments and intelligence.

Milley also discussed the recent deployment of U.S. military forces to Poland and Romania with their respective military leaders, according to the Pentagon.

-ABC News' Luis Martinez

While tens of thousands of Ukrainians were able to flee the country via humanitarian corridors on Thursday, those still in Mariupol have not been able to leave, as the city is under total siege by Russian forces.

A second attempt for residents to leave on Thursday was abandoned as the Russian military continued to bombard the Mariupol and the humanitarian passages surrounding it, Ukrainian President Volodymyr Zelenskyy said during a press briefing Thursday.

About 300,000 are suffering from lack of water, power and food, said Iryna Vereshchuk, a vice prime minister helping oversee the evacuations. Russia also destroyed and mined surrounding cities, Vereshchuk said.

Albert Khomyak, who lived in Mariupol's eastern outskirts, told ABC News that Russian troops have now entered their street.

"They are trying to destroy Mariupol, that way to destroy the resistance," he said.

Khomyak was somehow able to drive with his family in their own car out of the city, he said.

About 16,000 people are estimated to have evacuated Ukraine on Thursday now that the corridors near the northern Sumy region and some of the towns just northwest of Kyiv have been firmed up, Russian officials said.

But the ceasefires that allow these evacuations are temporary and very imperfect, and sporadic firing still happens throughout them.

The ceasefires then end at 9 p.m., when full-scale offensive attacks from Russia begin again.

-ABC News' Fidel Pavlenko and Natalie Vikhrov

The soaring gas prices Americans are experiencing are a result of the "stand" being taken for democracy in Ukraine, Rep. Raul Ruiz, D-Calif., said in a press conference Thursday from Philadelphia, where a retreat for House Democrats is ongoing.

Ruiz recast rising gas prices in a different light, saying they are part of the fight for democracy and in support of Ukraine against Russia.

"We have to ask ourselves, what is it we stand for as a democracy?" Ruiz, the chair of the Congressional Hispanic Caucus, said. "To protect other democracies, and to protect us from autocracies and to show solidarity? What is the price that we as individuals will want to contribute in solidarity with the Ukrainian people?"

Ruiz said his family are among the millions of Americans feeling the effects of the record-high prices.

"I, for one, am going to pay that price," Ruiz said. "If it means paying a few extra cents at the gas tank, then we're willing to pay that."

-ABC News' Ben Siegel

A senior U.S. defense official said Thursday that the U.S. has seen "indications" of Russia using a potential false flag operation about U.S.-Ukrainian biochemical weapons development as a pretext for the potential use of "these kinds of agents in an attack."

"We have picked up indications that the Russians could be making these claims, these false claims, about us and Ukrainian work in bio-defense, as a way of creating a pretext of their own to perhaps use these kinds of agents in an attack,” said the official.

On Thursday, there was a united front from the White House, State Department and the Pentagon rejecting Russian claims that the U.S. and Ukraine are developing these weapons, noting that Russia has previously used false flags and claims to justify the use of the very weapons they’ve made claims about.

When asked what prompted the U.S. to go public again with intelligence about a potential Russian false flag operation, a senior U.S. defense official would only describe "indications" of potential use.

"We're doing this because the Russians and the Chinese felt somehow important for them to put out a bunch of lies. They just flat out lied,” said the official. “We believe that this is potentially some sort of pre-textual argument that they are creating. We have indications of that."

The official added, “I won't get into how long we've had these indications or what the sources are thinking can understand that. But we believe their public narrative could be used, being used as a way to create that, that pretext."

When asked if that meant the U.S. had seen the movement of chemical or bio weapons near Ukraine or into Ukraine the official stuck to "indications". “I have to leave it with you with indications and [I’m] not going to be at liberty to go in more detail than that today.”

A senior defense intelligence official added that the U.S. assesses that Russia maintained an offensive bio-weapons program in violation of current conventions and no issues have been noted about Ukraine.

Ukraine’s intelligence agency also warned on Thursday that there are signs Russia might be planning to blow up a huge ammonia stockpile in eastern Ukraine as a false flag.

-ABC News' Luis Martinez

The Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler