US Vice President Joe Biden called Ukraine's President Viktor Yanukovych on Thursday to press for a peaceful end to its deadly crisis, as Washington issued a veiled threat of sanctions.

Before announcing the call, the White House had put the blame for the violence squarely on government forces in Kiev, but did welcome news of talks between opposition leaders and Yanukovych.

"The vice president urged President Yanukovych to take steps to end violence and to meaningfully address the legitimate concerns of peaceful protesters," the White House said in a statement.

"While emphasizing that violence by any side is not acceptable, the vice president underscored that only the government of Ukraine can ensure a peaceful end to the crisis and further bloodshed would have consequences for Ukraine's relationship with the United States."

Washington said it was considering whether to impose sanctions on the Ukrainian government over the violence, which it says is the result of a refusal to take the grievances of the protesters seriously.

"We are deeply concerned by the violence," White House spokesman Jay Carney said. "We welcome the news that President Yanukovych is meeting directly with opposition leaders."

State Department spokeswoman Marie Harf meanwhile delivered a veiled threat that Washington could resort to sanctions.

"We're looking at a a variety of policy options, including sanctions," she said.

"We're not eager to do so, but we need to calibrate our policy based on the situation on the ground, and we hope the government takes advantage of this opportunity."

Earlier, German Chancellor Angela Merkel said that it was not yet appropriate to slap sanctions on the Kiev government, but also called Yanukovych to urge a return to order.

Activists say five protesters have died in clashes in recent days, marking a new peak in tensions after two months of protests over the government's failure to sign a deal for closer integration with the European Union.