Volodymyr Zelenskiy, the president of Ukraine, will address Congress on Wednesday in what could prove his most powerful plea yet for the west to take a tougher line against Vladimir Putin.

Zelenskiy is expected to use the virtual address to urge members of the House of Representatives and Senate to intensify pressure on Joe Biden to allow the transfer of MiG-29 fighter jets from Poland.

The House speaker, Nancy Pelosi, and Senate majority leader, Chuck Schumer, said in a joint letter to members: “The Congress, our country and the world are in awe of the people of Ukraine, who have shown extraordinary courage, resilience and determination in the face of Russia’s unprovoked, vicious and illegal war.”

They added: “The Congress remains unwavering in our commitment to supporting Ukraine as they face Putin’s cruel and diabolical aggression, and to passing legislation to cripple and isolate the Russian economy as well as deliver humanitarian, security and economic assistance to Ukraine.

“We look forward to the privilege of welcoming President Zelenskiy’s address to the House and Senate and to convey our support to the people of Ukraine as they bravely defend democracy.”

Zelenskiy, who will speak at 9am Washington time on Wednesday, has been seeking to drum up support with video briefings of foreign audiences. Last week he received a standing ovation from the British parliament and echoed William Shakespeare (“The question for us now is: ‘To be or not to be’”) and Winston Churchill (“We will fight in the forests, in the fields, on the shores, in the streets”).

On Tuesday, the TV actor and comedian turned resistance leader, who has proved adept at communications under siege, is scheduled to address the Canadian parliament in Ottawa. He is also due to speak to Israel’s parliament at some stage.

On 5 March, dressed in a military-green T-shirt and seated beside a Ukrainian flag, Zelenskiy spoke to more than 280 members of the House and Senate in a video call. He is said to have made a “desperate plea” for aircraft to fight Russian invaders.

Most members of Congress back the White House’s refusal to attempt to impose a “no-fly zone” that could entail US pilots firing on Russians and trigger a wider conflict.

Chris Murphy, chairman of the Senate appropriations homeland security subcommittee, told the Hill: “This is the most dangerous moment since the Cuban missile crisis. We have never been this close to direct conflict with Russia.

“We made the right decision to openly support the Ukrainians but we just should understand the unprecedented moment that we’re living in today where we’re openly funding war against a nuclear power.”

But there is a growing split over Poland’s offer to send Soviet-style MiG-29 fighter jets, which Ukrainian pilots are capable of flying, to Ukraine via a US airbase in Germany.

The White House and Pentagon have rejected the proposal, wary that an increasingly reckless Putin could perceive it as escalatory and saying it raised “serious concerns” for the entire Nato alliance. Republicans and some Democrats say Zelenskiy’s request should be met.

Mitt Romney, a Republican senator from Utah, said last week: “He has asked us for aircraft – specifically MiGs. We need to get him those MiGs. It is a bipartisan message.”

Rob Portman, a Republican senator from Ohio visiting the Ukraine-Poland border, told CNN: “What we’ve heard directly from the Ukrainians is they want them badly. They want the ability to have better control over the skies in order to give them a fighting chance. I don’t understand why we’re not doing it.”

The Democratic senator Amy Klobuchar, from Minnesota, was also on the visit. She said she had spoken to Biden “about 10 days ago” about the fighters, adding: “I’d like to see the planes over there.”

The Democratic-controlled Congress approved $13.6bn in humanitarian and security aid to Ukraine last Thursday, as part of a $1.5tn spending bill that funds US government operations through 30 September.

The US and allies have imposed broad sanctions on Russia after the invasion of Ukraine on 24 February. Biden has announced a US ban on Russian oil imports, seen as politically risky amid soaring gas prices.

Last Friday the president took more steps to punish Russia economically, targeting trade and shutting down development funds while announcing a ban on imports of Russian seafood, vodka and diamonds. On Saturday he authorised $200m in additional military equipment for Ukraine.

About 59% of Americans believe Biden has been making the right decisions when it comes to the situation in Ukraine, including more than one in three Republicans, according to Navigator Research. However, asked whether they approve of Biden’s handling of the issue, Americans are more polarised, with 49% disapproving and 43% approving.

Biden’s predecessor as president, Donald Trump, again refused to condemn Putin at a rally in South Carolina on Saturday.

“It happens to be a man that is just driven, he’s driven to put it together,” Trump said, while claiming the war would never have happened if he was still in the White House.

On Monday a fourth round of talks between Ukraine and Russia were held via videoconference amid deadly air strikes in the Ukrainian capital, Kyiv. At the weekend, Russian airstrikes killed 35 people at a military base near Yavoriv, outside Lviv – perilously close to the frontier with Poland, a Nato member.

In a video address, Zelenskiy warned: “If you do not close our sky, it is only a matter of time before Russian missiles fall on your territory, on Nato territory, on the homes of Nato citizens.”

He urged Nato to impose a no-fly zone – a request he is likely to repeat to Congress on Wednesday.