The cost of rebuilding Ukraine following Russia's invasion could reach $750 billion and rich Russians should help pay the bill, Ukrainian Prime Minister Denys Shmygal said on Monday.

"We believe that the key source of recovery should be the confiscated assets of Russia and Russian oligarchs," he told a conference in the Swiss city of Lugano, citing estimates that frozen Russian assets were worth $300-$500 billion.

"The Russian authorities unleashed this bloody war. They caused this massive destruction and they should be held accountable for it."

Russia has said its "special military operation" in Ukraine is meant to demilitarise its southern neighbour and protect Russian speakers from what it calls nationalists.

Ukraine and its Western allies say this is a baseless pretext for flagrant aggression that aims to seize territory.

Shmygal's views on Monday was echoed by British Foreign Secretary Liz Truss who said Russia needed to be held accountable for the damage caused by its "appalling war," while Kyiv also needed help to rebuild its shattered economy.

"We are looking at options for the deployment of Russian assets," Truss told Reuters on the sidelines of the conference.

"At the same time we are doing what we can to get the Ukrainian economy restarted - getting those grain exports out of Odesa, making sure we are supporting Ukrainian industry and business to get going," she said.

Shipping insurance and adequate weaponry to protect ports exporting grain were among the areas being looked at, Truss said.

Also addressing the Ukraine Recovery Conference, European Commission President Ursula von der Leyen said the European Union would establish a central platform to coordinate rebuilding efforts and also help to cement Ukraine's status as a candidate for EU membership, which the EU agreed last month.

"Since the beginning of the war, the European Union has mobilised around 6.2 billion euros ($6.48 billion) in financial support," von der Leyen said. "And... more will come. We will engage substantially in the mid- and long-term reconstruction."

The platform will map investment needs and channel resources, von der Leyen said.

It will bring together countries, the private sector, civil society, as well as international organisations, such as the European Bank for Reconstruction and Development and the European Investment Bank.

The European Investment Bank, the lending arm of the European Union, is proposing a funding structure previously used during the COVID-19 pandemic to help rebuild Ukraine.

Ukraine's President Volodymyr Zelenskiy hailed the reconstruction efforts.

"To rebuild Ukraine is to restore the principles of life, to restore the spaces of life, to restore what makes people human," he said by videolink.

($1 = 0.9570 euros)