US President Joe Biden on Thursday was to propose a huge $33 billion package for arming and supporting Ukraine in its war against Russia, as well as using luxury assets stripped from Russian oligarchs to compensate the battered pro-Western country.

Speaking in the White House, Biden was set to announce the next phase in the already unprecedented effort to punish Russia for its invasion and to rush Western weaponry to Ukrainian troops.

The bulk of the proposed $33 billion package will be "$20 billion in military and other security assistance. This means weapons and ammunition flowing to the Ukrainian people," a senior US official said.

A further $8.5 billion in economic aid will "help the government of Ukraine respond to the immediate crisis," while some $3 billion is proposed to fund humanitarian assistance and address the global food supply price shock resulting from Russia's onslaught against Ukraine, a major wheat exporter.

The proposed package also includes funding to address economic disruptions in the United States and elsewhere, ranging from the impact on food supplies to availability of critical components used in high-tech manufacturing.

Congress needs to approve the request and while both Republicans and Biden's Democratic party have signalled they are keen to keep backing Ukraine, an unrelated dispute over Biden's request for billions of dollars in new Covid pandemic funding threatens to complicate the approval process.

- Going after oligarchs -

In parallel with the military build-up -- which began cautiously with mostly defensive infantry weapons, but now includes heavy artillery and tank-killing drones -- Washington is leading a Western sanctions onslaught designed to isolate Russia and pressure President Vladimir Putin.

Biden is also ratcheting up pressure on the oligarch billionaires who have already come under heavy sanctions for supporting Putin.

Under his proposal, "the proceeds of forfeited kleptocratic property" will be used to compensate Ukraine for the "harms of Russian aggression," the White House said in a statement.

To date, European Union allies have frozen more than $30 billion in Russian assets, including almost $7 billion in luxury goods belonging to oligarchs, including yachts, art, real estate and helicopters, the White House said.

The United States has "sanctioned and blocked vessels and aircraft worth over $1 billion, as well as frozen hundreds of millions of dollars of assets belonging to Russian elites in US accounts," said the statement. One of the latest seizures was of a $90 million superyacht belonging to Russian billionaire Viktor Vekselberg.

The legislative package being proposed by Biden would also tighten the legal squeeze on oligarchs as they try to hide their funds.

One proposal is to allow seizing of property used to evade sanctions. Another is to expand the arsenal used by US prosecutors, doubling the amount of time they are allowed to pursue money laundering investigations from five to 10 years, and applying anti-racketeering laws used to tackle organized crime to sanctions evasion.