The United States and Britain says they are determined to track down billions of dollars of Ukrainian assets allegedly looted under the regime of deposed president Viktor Yanukovych.

US Attorney General Eric Holder and British Home Secretary Theresa May told an international conference on asset recovery that those responsible would be held accountable.

"There should be no mistake, we are determined in our efforts to be successful," Holder told a press conference in London at the start of the two-day forum.

"We are determined to hold accountable those who were responsible for the theft of these Ukrainian assets and we are also determined to ensure that those assets are returned to the Ukrainian people."

Ukraine's general prosecutor Oleh Makhnitskyi told the forum that Kiev has already identified stolen assets totalling at least 35 billion Ukrainian hryvnias ($A3.25 billion).

He expected the eventual total to amount to tens of billions of dollars.

Makhnitskyi described the Yanukovych regime as an "organised criminal group" whose tentacles reached throughout the administration.

"The new government was set up and we found that our treasury was empty and the funds were misappropriated," he said.

Yanukovych was ousted in February following a series of massive protests after he decided to scrap an agreement with the European Union in favour of closer ties with Russia.

He fled Ukraine for Russia.

Ukrainian Interior Minister Arsen Avakov, senior government officials, prosecutors and representatives from financial centres and international organisations around the world are also attending the conference, Britain's Home Office said.

Britain's Serious Fraud Office announced on the eve of the conference that it had launched a money-laundering investigation into possible corruption in Ukraine and frozen $US23 million in assets.

May said officials from Britain's National Crime Agency and Crown Prosecution Service have already travelled to Ukraine to offer their assistance.

"I think this event will help to set a new benchmark for the international community," she said.

British Foreign Secretary William Hague said on Monday he will visit Ukraine next week in a show of support for Kiev.