Ukrainian President Volodymyr Zelenskyy on Wednesday sought more help from the United States to defend against Russia's aggression during an online appearance before Congress, saying President Joe Biden should become the world's "leader of peace" as he rallies other countries to stand by Kyiv.

Zelenskyy, who has also made virtual addresses to parliaments in Europe and Canada, reiterated his call for air support to protect the skies over Ukraine as the country is under bombardment. The Biden administration has continued to turn down the request, citing fear the move could prompt a direct war between the United States and Russia.

"Today the Ukrainian people are defending not only Ukraine, we are fighting for the values of Europe and the world, sacrificing our lives in the name of the future," said the 44-year-old comedian-turned-president who has emerged as a heroic war leader.

Ukrainian President Volodymyr Zelenskyy delivers a virtual address to Congress at the U.S. Capitol on March 16, 2022 in Washington as Ukraine continues to defend itself from an ongoing Russian invasion. (Getty/Kyodo)

"As the leader of my nation, I'm addressing President Biden. You are the leader of the nation, of your great nation. I wish you to be the leader of the world," he said, adding, "Being the leader of the world means to be the leader of peace."

During his speech, a video was aired to show parts of Ukraine being reduced to ruins, children becoming victims of the attacks and scenes of dead bodies lying on the ground, ending with an appeal to "close the sky over Ukraine."

The Biden administration has been providing security assistance to the former Soviet republic, but it has remained cautious about any moves that could be deemed "escalatory," such as the creation of a so-called no-fly zone, which officials say would essentially mean U.S. aircraft "shooting down Russian planes and them potentially shooting back."

Zelenskyy raised the question with U.S. lawmakers as to whether a no-fly zone is "too much" to ask for, saying what Ukraine is facing now is similar to the U.S. experience in two of the most significant attacks in its history -- the Japanese assault on Pearl Harbor in 1941, which drew the United States into World War II, and the Sept. 11 terror attacks in 2001.

As an alternative, he called for "powerful" aircraft to fight back against Russian military and anti-aircraft missile systems.

The Biden administration has also been against the idea of helping to transfer Russian-made MiG warplanes into Ukraine, while repeatedly ruling out putting U.S. troops on the ground in Ukraine, warning that being drawn into a direct conflict with nuclear-armed Russia would result in "World War III."

Russia launched a military attack on Ukraine on Feb. 24 after asserting that its own security was under threat from the North Atlantic Treaty Organization's eastward expansion and the possibility of Kyiv joining the alliance.

The move has triggered condemnation from around the world and a wave of tough economic sanctions from the United States, Europe and U.S. allies and partners including Japan to isolate Russia from the global financial system.

Zelenskyy told the U.S. Congress that he would like to see the United States impose sanctions against "all politicians" in Russia who remain in office and do not sever ties with those responsible for the aggression. He also said all American companies should leave the Russian market, which is "flooded with our blood."

Following the Ukrainian leader's appeal, Biden announced an additional $800 million in security assistance, which comes from a massive spending bill he signed into law on Tuesday. He said the package includes hundreds of anti-aircraft systems and small arms and drones.

The latest announcement nearly doubles the total security assistance to Ukraine since the beginning of the Biden administration to more than $2 billion, the U.S. government said.

Biden also stepped up his criticism of Russian President Vladimir Putin, acknowledging him as a "war criminal" to reporters, according to U.S. media.

White House Press Secretary Jen Psaki said Biden was "speaking from his heart" as the world sees the "barbaric actions by a brutal dictator through his invasion of a foreign country." But she also noted that investigations are under way on whether Russia can be judged to have engaged in acts that would constitute war crimes.

The State Department has been looking into whether Russia has perpetrated deliberate attacks on civilians or other atrocities, documenting the information so that relevant institutions can make a judgement.