Ukraine's leader Volodymyr Zelensky made a desperate appeal for help against Russia to the US Congress on Wednesday, and within hours of the speech, President Joe Biden responded by promising $1 billion in new weapons to fight "war criminal" Vladimir Putin's invading army.

The Ukrainian president's video address from embattled Kyiv, bolstered by a montage of horrific TV footage of Ukrainian civilians under Russian assault, delivered an emotional gut-punch to US lawmakers.

Ukrainian Ambassador to the United States Oksana Markarova (L) speaks with US House speaker Nancy Pelosi prior to President Volodymyr Zelensky's address to the US Congress
Ukrainian Ambassador to the United States Oksana Markarova (L) speaks with US House speaker Nancy Pelosi prior to President Volodymyr Zelensky's address to the US Congress POOL via AFP / SARAH SILBIGER

Bearded and dressed in a military green T-shirt, the Ukrainian leader invoked September 11 and Pearl Harbor as he pleaded again for a Western-enforced no-fly zone to bar Russian warplanes, something Biden has rejected as almost certainly leading to what he says would be "World War III."

More than three weeks into a war that has claimed hundreds of civilian lives and turned more than three million Ukrainians into refugees, Zelensky said his country was battling "a terror that Europe has not seen... for 80 years."

"Remember Pearl Harbor, terrible morning of December 7, 1941, when your sky was black from the planes attacking you," he said, recalling the air raid that brought the United States into World War II.

Ukrainian President Volodymyr Zelensky addresses the US Congress by videolink on March 16, 2022
Ukrainian President Volodymyr Zelensky addresses the US Congress by videolink on March 16, 2022 POOL via AFP / Drew Angerer

"Protect our sky," he said.

But in a nod to Western concerns, Zelensky added that an alternative to the no-fly zone would be more powerful weapons to allow Ukrainians to defend themselves.

Just hours later, Biden delivered his response: an extra $800 million in military aid, which added to another installment last week brings the latest deliveries to $1 billion -- and $2 billion since Biden took office.

Ukrainian Ambassador to the United States Oksana Markarova (L) speaks with US House speaker Nancy Pelosi prior to President Volodymyr Zelensky's address to the US Congress
Ukrainian Ambassador to the United States Oksana Markarova (L) speaks with US House speaker Nancy Pelosi prior to President Volodymyr Zelensky's address to the US Congress POOL via AFP / SARAH SILBIGER

The new batch includes 800 Stinger anti-aircraft systems, thousands of anti-tank weapons including 2,000 of the now-famously deadly Javelin rockets, 100 "tactical" drones, 20 million rounds of small arms ammunition and 25,000 sets of helmets and body armor.

Ukraine has already been given hundreds of Stingers, which can shoot down relatively low flying aircraft.

But Biden said that "at the request of President Zelensky, we have identified and are helping Ukraine acquire additional longer-range anti-aircraft systems."

Ukraine's leader Volodymyr Zelensky urged US President Joe Biden to be 'the leader of peace'
Ukraine's leader Volodymyr Zelensky urged US President Joe Biden to be 'the leader of peace' AFP / Nicholas Kamm

A US military source, who spoke on condition of anonymity, said the weapons system under discussion was the S-300, a sophisticated missile that is of Russian design but owned by some European NATO members and seen as easy to integrate into Ukraine's military.

The drones mentioned by Biden are a US weapon called Switchblade, which has the ability to loiter over a target, before plunging down like a bomb, the military source said.

Ukrainian Ambassador to the United States Oksana Markarova (L) speaks with US House speaker Nancy Pelosi prior to President Volodymyr Zelensky's address to the US Congress
Ukrainian Ambassador to the United States Oksana Markarova (L) speaks with US House speaker Nancy Pelosi prior to President Volodymyr Zelensky's address to the US Congress POOL via AFP / SARAH SILBIGER

Biden also ramped up the rhetorical offensive against the Kremlin, telling reporters for the first time that Putin "is a war criminal." Putin's spokesman called this "unacceptable and unforgivable."

Emotions were running high in Washington after what some called the most dramatic speech by a foreign leader to Congress since Britain's prime minister Winston Churchill in 1941.

But it was an iconic speech by US civil rights leader Martin Luther King Jr that Zelensky, a TV comedian turned wartime leader, channeled in his desperate plea for assistance against Russia's bombs.

"I have a dream, these words are known to each of you today I can say. I have a need, I need to protect our sky. I need your decision, your help," he said.

Zelensky spoke in Ukrainian initially but ended his speech in English, addressing Biden directly with an appeal for him to be the world's "leader of peace."

Zelensky's appearance -- greeted by a standing ovation in the US Capitol complex -- came less than a week after lawmakers green-lit nearly $14 billion in humanitarian and military aid for the war-torn nation.

Biden, who will meet next week with NATO and European allies in Brussels, is under growing pressure as he tries to balance assistance for Ukraine against fears of wider war with Russia.

The issue of weapons to Ukraine is more fraught.

Short of backing a no-fly zone over Ukraine, many lawmakers support Zelensky's pleas for the United States to broker the transfer of MiG fighter jets from Poland. But Biden has also ruled out US involvement in such a transfer, saying it would again amount to a dangerous escalation with Moscow.

"The idea that we're going to send in offensive equipment and have planes and tanks and trains going in with American pilots and American crews -- just understand, don't kid yourself, no matter what you all say, that's called World War III," Biden told House Democrats last week.