Joe Biden has called for a giant $33bn package of military and economic aid to Ukraine, more than doubling the level of US assistance to date, in an emphatic rejection of Russian threats of reprisals and escalation.

A few hours after Biden spoke, Kyiv was shaken by two powerful cruise missile strikes, while the UN secretary general, António Guterres, was visiting the Ukraine capital following a meeting with Vladimir Putin in the Kremlin.

A senior Ukrainian presidential aide, Mykhailo Podolyak, called the attack a “postcard from Moscow” and asked why Russia still had a seat on the UN security council.

Biden asked Congress to give immediate approval for spending that would include over $20bn in military aid, involving everything from heavy artillery and armoured vehicles to greater intelligence sharing, cyber warfare tools and many more anti-tank and anti-aircraft missiles.

Biden also requested $8.5bn in economic aid to Kyiv and $3bn in humanitarian relief, as well as funds to help increase US production of food crops and strategic minerals to offset the impact of the war in Ukraine on global supplies.

The total of $33bn is more than twice the last supplemental request approved by Congress in March and dwarfs the entire defence budget of Ukraine and of many other countries. The US president said it was aimed at helping Ukraine repel the renewed Russian offensives in the east and south of the country, but also to transition to assuring the nation’s longer-term security needs.

On the same day, Congress agreed to update the 1941 lend-lease legislation with which Franklin D Roosevelt sought to help Britain and other allies fight Nazi Germany. The updated law is intended to make it easier for the US to provide military equipment to Ukraine.

It comes in the face of Russian warnings that increased western weapons supplies to Ukraine would endanger European security, that western intervention could bring instant Russian reprisals and raise the risk of nuclear conflict.

Making the case for western aid, Biden argued that on the contrary, if Putin was not stopped in Ukraine he would continue to threaten global peace and stability.

The president framed the request principally in terms of defending Ukraine, and did not explicitly repeat the declaration earlier this week by his defence secretary, Lloyd Austin, that one of US aims in Ukraine was to weaken Russia to stop it attacking other countries.

“Despite the disturbing rhetoric coming out of the Kremlin, the facts are plain for everybody to see. We’re not attacking Russia. We’re helping Ukraine defend itself against Russian aggression,” Biden said. But he added the cost involved was “a small price to pay to punish Russia and aggression, to lessen the risk of future conflicts”.

“Throughout our history, we’ve learned that when dictators do not pay the price for their aggression, they cause more chaos and engage in more aggression,” he said. “The threats to America and the world keep rising. We can’t let this happen.”

The new military assistance the congressional funding will finance will include:

• More artillery and armored vehicles, as well as anti-tank missiles and anti-aircraft systems.

• Help to build up Ukraine’s cyber warfare capabilities.

• More intelligence sharing.

• Support to increase Ukraine’s ability to produce munitions and strategic minerals.

• Assistance in clearing landmines and other explosives and in Ukraine’s defence against chemical, biological and dirty bomb attacks.

• A further buildup in the US military presence on Nato’s eastern flank.

The Kremlin’s official spokesperson, Dmitry Peskov, warned on Thursday that an increased western supply of heavy weapons to Kyiv would endanger European security.

“The tendency to pump weapons, including heavy weapons, into Ukraine, these are the actions that threaten the security of the continent, provoke instability,” Peskov said.

The day before, Vladimir Putin had threatened a “lightning fast” response to western intervention in Ukraine, adding: “We have all the weapons we need for this.”

His foreign minister, Sergei Lavrov, has accused the US and its allies of fighting a proxy war in Ukraine and warned of the rising danger of a nuclear conflict.

Biden rejected the accusation he was fighting a proxy war, describing the claim as part of the Kremlin’s domestic propaganda to explain the inability of Russian forces to achieve their goals.

“I think it’s more of a reflection, not of the truth, but of their failure,” the president said. He added: “No one should be making idle comments about the use of nuclear weapons.”

The package of proposals the administration is sending to Congress also includes measures to strengthen the hand of the justice department in pursuing Kremlin-aligned oligarchs seizing their assets and using the proceeds to support the Ukraine war effort.

Biden said the measures would allow for “expanded and expedited measures for investigating, prosecuting, and forfeiting assets of Russian oligarchs to be used for the benefit of Ukraine”.

“We’re going to seize their yachts and luxury homes and other ill-begotten gains of Putin’s kleptocracy,” he added.

The president made his announcement as the UN secretary general was visiting Ukraine, where he described the war as “an absurdity” in the 21st century.

Guterres was touring Borodianka on Thursday, where Russian forces are accused of massacring civilians before their withdrawal, on his first visit to Ukraine since the start of the invasion on 24 February, before talks with President Volodymyr Zelenskiy.

In nearby Bucha, where dozens of civilian bodies, some with their hands tied, were discovered this month, Guterres backed an investigation by the International Criminal Court into possible war crimes in Ukraine. “I appeal to the Russian Federation to accept, to cooperate with the ICC,” he said.

The humanitarian impact of the Russian invasion has been devastating. The UN refugee agency UNHCR said nearly 5.4 million Ukrainians had fled their country since the attack began, with more than 55,000 leaving in the past 24 hours. While the outflow has slowed significantly since March, it forecast that the conflict in Ukraine could produce 8.3 million refugees by the end of the year.

Maria Zakharova, Russia’s foreign ministry spokesperson, warned the west on Thursday to stop encouraging Ukraine to strike at targets inside Russian territory, saying it was “trying our patience”. Multiple targets, including fuel and ammunition depots, have been hit in Russian provinces bordering Ukraine in recent days.

“Such aggression against Russia cannot remain without an answer,” Zakharova said. “We would like Kyiv and western capitals to take seriously the statement that further provocation prompting Ukraine to strike against Russian facilities will be met with a harsh response from Russia.”

Podolyak, the Ukrainian presidential aide, defended the country’s right to strike inside Russia, saying: “Ukraine will defend itself in any way, including strikes on the warehouses and bases of the killers in Russia. The world recognises this right.”

Britain’s defence secretary, Ben Wallace, on Thursday also repeated the UK’s assertion that it was “legitimate under international law” for Ukrainian forces to target Russian logistics infrastructure, but he said such attacks were unlikely to use British weapons.

The US on Thursday accused Russia of planning fake independence votes to justify its conquest of Ukrainian territory, saying the Kremlin might attempt “sham referenda” in southern and eastern areas it had captured using “a well-worn playbook that steals from history’s darkest chapters” and must “never be recognised as legitimate”.