Lithuania on Tuesday confirmed signs of torture on a prominent Ukrainian anti-government activist and urged an independent probe into the crime which could breach a related United Nations convention.

"Dmytro Bulatov...has clear signs of long term torture and cruel treatment on his body," a Lithuanian foreign ministry said in a statement.

Twelve people were killed when a train ploughed into a minibus in northern Ukraine on Tuesday, emergency officials said.

Five people have been hospitalized following the accident, which happened in the Sumy region close to the border with Russia.

Ukrainian President Viktor Yanukovych could call early elections to end mass anti-government unrest, a top lawmaker said Tuesday, as protest leaders demanded curbs to presidential powers in a stormy parliamentary debate.

EU foreign policy chief Catherine Ashton meanwhile was set to arrive in Kiev to press for a resolution to the crisis, as Europe and the United States discussed a possible financial aid package to Ukraine in exchange for democratic reforms.

European Union foreign policy chief Catherine Ashton returns to Ukraine on Tuesday in a fresh bid to help bring an end to the country's crisis, her office said.

Ashton is expected to begin talks with the opposition and authorities after arrival and continue meetings on Wednesday, said Ashton's spokeswoman Maja Kocijancic. She refused to give further details.

A majority of Ukrainians back the opposition's demand of early elections but are about evenly split between supporters and opponents of an association agreement with the European Union, a poll showed on Monday.

The Internet survey by the TNS agency in Kiev found 51.3 percent of its 1,008 respondents -- all town or city residents -- favored bringing President Viktor Yanukovych's term to an end and calling early elections.

Russia on Monday urged Ukraine's opposition to stop issuing "threats and ultimatums" after it appealed for Western aid and urged President Viktor Yanukovych to accept curbs on power.

The statement from Moscow, which has publicly supported Yanukovych in his confrontation with the opposition, came after Ukrainian protest leaders rallied tens of thousands in central Kiev on Sunday.

Ukrainian President Viktor Yanukovych is unlikely to move the army against protesters despite opposition warnings about an imminent intervention -- mainly because the loyalty of rank and file soldiers could be in doubt, analysts said.

"The core of the army is made up of young people who grew up in an independent Ukraine," said Valentyn Badrak, director of the Research Center for the Army, Demilitarization and Disarmament in Kiev.

A Ukrainian protester whose account of torture has shocked Europe arrived in Vilnius late Sunday, hours after a Kiev court ruled that he could leave the country for treatment.

Dmytro Bulatov left Ukraine following intense pressure by Western leaders after he appeared on television, his face swollen and caked in blood, and said he had been kidnapped and tortured over his role in protests that have rocked the country.

A protester who said he was kidnapped and tortured was flown out of Ukraine on Sunday after a dramatic stand-off between opposition leaders and police at the clinic treating him and a last-minute ruling by a Kiev court.

Dmytro Bulatov, one of the activists behind the Avtomaidan movement that has helped spearhead protests, was driven to the airport by ambulance and took a flight to Riga.

Tens of thousands of protesters rallied in Ukraine on Sunday in a bid to wring new concessions from President Viktor Yanukovych, buoyed by pledges of support from Europe and the United States.

More than 50,000 people could be seen on Kiev's barricaded Independence Square -- that has become the epicenter of a two-month protest movement -- and thousands more were arriving.