The White House said Thursday it was "outraged" that Ukrainian forces had turned automatic weapons on demonstrators as a short-lived truce shattered in Kiev.

In a statement, White House press secretary Jay Carney called for the withdrawal of government forces from the front lines in the Ukrainian capital, but did not specify what consequences there would be from Washington after its previous, repeated calls for an end to violence were ignored.

"We are outraged by the images of Ukrainian security forces firing automatic weapons on their own people," Carney said.

"We urge President (Viktor) Yanukovych to immediately withdraw his security forces from downtown Kiev and to respect the right of peaceful protest, and we urge protesters to express themselves peacefully."

Carney urged the Ukrainian military not to get involved in a conflict that he said should be resolved by political means and said Washington would work with its allies to hold accountable those responsible for violence.

The Pentagon echoed the White House's comments and expressed disappointment that Defense Secretary Chuck Hagel's attempts to speak to his Ukrainian counterpart, Pavel Lebedev, had been rebuffed.

"We've been trying now for the past several days, as the violence escalated, to reach the ministry of defense," Pentagon spokesman Rear Admiral John Kirby told reporters.

"So far, the ministry of defense has been unresponsive to our requests," he said, adding that "they haven't taken the call."

The Ukrainian president replaced the head of the army's general staff on Wednesday, but so far the military has not been implicated in the violence gripping the capital Kiev, Kirby said.

"The reports we are receiving right now from our embassy personnel in Kiev indicate that the armed forces are being used at this time to protect military facilities, including weapons and ammunition storage facilities, and are not involved in the violence against protesters," he said.

"We certainly hope that remains the case."

On Wednesday, Obama warned Ukraine's government of "consequences" if it did not stop turning violence on peaceful protestors.

The administration also put 20 top Ukrainian officials on a visa blacklist and threatened further sanctions.

But fresh violence erupted Thursday, with raging street battles between protestors and security forces.

More than 60 protesters died from gunshot wounds, according to an opposition medic, making the day the bloodiest since Ukraine's independence.