The United States and Britain said Tuesday they were determined to track down billions of dollars of Ukranian 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 ($3 billion, 2.1 billion euros).

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 $23 million (17 million euros) 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 Monday he will visit Ukraine next week in a show of support for Kiev.

Holder said countries accused of dragging their feet over Ukrainian assets would have to choose sides.

"Do you stand with the Ukrainian people or do you stand with those who have robbed and stolen from the Ukrainian people? It's a simple decision," he said.

Several countries are helping Ukrainian-led investigations into alleged corruption and money laundering by members of Yanukovych's government.

Swiss authorities have ordered a freeze on the assets of both Yanukovych and his multi-millionaire son Oleksandr, as well as 18 other former ministers and officials.

The hunt for Ukraine's missing money comes as Washington and Brussels harden their economic front against Russia over the crisis.

The European Union said Tuesday it had targeted Russia's armed forces chief of staff and its military intelligence chief in its latest round of punitive measures against 15 individuals.

The White House on Monday slapped sanctions on seven Russian officials and 17 companies close to President Vladimir Putin.