Explained: What is a vacuum bomb which Ukraine says Russia has used

Ukraine's ambassador to the U.S., Oksana Markarova, met Monday on Capitol Hill with House and Senate lawmakers as the Russian military assault on Ukraine went into its fifth day.

Markarova met Monday afternoon with the bipartisan House Ukraine Caucus, followed by a briefing later in the evening to the Senate Ukraine Caucus.

Speaking to reporters in between the two meetings, Markarova said Monday was "especially difficult" in her country's war with Russia. She said Russia is stepping up its attacks against Ukraine's innocent civilians and accused Russian President Vladimir Putin of using a vacuum bomb against residential areas.

"They used the vacuum bomb today, which is actually prohibited by the Geneva convention," Markarova said after briefing members of House.

"The devastation that Russia is trying to inflict on Ukraine is is large. But we Ukrainians will resist," she said. "We are defending our home. We do not have any other option. We will not get tired. We will not stop. We will not surrender."

White House press secretary Jen Psaki told reporters earlier in the day the administration can't confirm reports of Moscow's use of a vacuum bomb. But she said, if it is true, "it would potentially be a war crime."

Russian forces shelled Ukraine's second-largest city on Monday, rocking a residential neighborhood, and closed in on the capital, Kyiv, in a 17-mile convoy of hundreds of tanks and other vehicles, as talks aimed at stopping the fighting yielded only an agreement to keep talking.

Amid ever-growing international condemnation, Russia found itself increasingly isolated five days into its invasion, while also facing unexpectedly fierce resistance on the ground in Ukraine and economic havoc at home.

Markarova said Ukraine is urging the United States and all Western allies for more weapons to fight Russia and also for continued and stepped up sanctions.

"We need more weapons. I mean, we're not asking anyone to fight for us. We are defending our country ourselves. But we need all the support that all civilized world can give us to actually continue effectively fighting it," she said.

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