Here are the latest developments in Ukraine.

President Volodymyr Zelensky of Ukraine is scheduled to speak via video to the U.S. Congress on Wednesday, in another high-stakes appeal for help a day after a dramatic meeting with three European leaders in Kyiv as fires blazed in the wreckage from Russia’s relentless bombardment.

Mr. Zelensky is expected to amplify his pleas for more military aid and to increase the pressure on the United States and its allies, which have so far been reluctant to agree to Ukraine’s request for Polish MIG fighter planes.

President Biden is planning to announce $800 million in new security assistance to Ukraine on Wednesday shortly after Mr. Zelensky’s speech, according to White House officials. His administration has already pledged aid in the hundreds of millions, but Mr. Zelensky says Ukraine needs far more.

Ukrainian officials plan to present the United States with a list of military equipment they need, including armed drones and mobile air-defense systems, as Mr. Zelensky addresses Congress, according to two European diplomats briefed on the request. Beyond the MIG fighter planes, Ukrainian officials have crafted a list of additional matériel the Biden administration might be more willing to give, one of the diplomats said.

Mr. Zelensky has tried to rally international support as Ukraine faces a devastating barrage from Russian forces, largely on its own.

“This is their attempt to annihilate the Ukrainian people,” Mr. Zelensky said in a video address to the Canadian Parliament, repeating his plea for NATO to enforce a no-fly zone over the country. “It is an attempt to destroy our future, our nation, our character.”

Here’s a look at other developments:

The risky visit by the prime ministers of Poland, the Czech Republic and Slovenia unfolded in tight secrecy as they crossed the Ukrainian border by train after dawn. In a statement after the meeting, Mr. Zelensky said the trio “fear nothing,” adding, “I am sure that with such friends, with such countries, with such neighbors and partners, we will really be able to win.”

President Vladimir V. Putin of Russia disparaged the second consecutive day of negotiations with Ukraine, undercutting the faint glimmers of hope raised from talks the day before that both sides were looking for a way to halt the war.

Mr. Biden announced his own plans to travel to Europe next week to showcase the unity of the NATO alliance in the face of Russian aggression.