Russia-Ukraine updates: Putin says 'certain positive movements' in negotiations

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

President Joe Biden declared that Putin will never conquer Ukraine on Thursday during a meeting with Colombian President Iván Duque Marquez at the White House.

"Ukraine will never never be a victory for Putin," Biden said.

Biden thanked Marquez for condemning Russia’s invasion of Ukraine "and the atrocities they're committing" and designated Colombia as a major non-NATO ally.

The decision will allow Colombia to be eligible for certain benefits in areas of defense trade and security cooperation, while also providing military and economic privileges.

"It’s just not just the threat to Europe and Ukraine, but it’s the threat to international peace and stability," Biden said. "And so Columbia -- Columbia's voice is making it clear that you share that view and that there's a lot more at stake than just the immediate undertaking."

Biden also announced that the U.S. will give 2 million more COVID-19 vaccine doses to Colombia to help vaccinate Colombia’s migrant population.

-ABC News' Justin Gomez