The White House on Wednesday called the deadly violence on the streets of the Ukrainian capital "completely outrageous" and renewed its appeal to President Viktor Yanukovych to de-escalate the situation.

Clashes in Kiev between police and anti-government protesters that have claimed 26 lives are "completely outrageous" and "have no place in the 21st century," Deputy National Security Advisor Ben Rhodes told reporters aboard Air Force One.

"The fact of the matter is we have made very clear to the Ukrainian government that it is their responsibility to allow for people (to) protest," Rhodes said.

"We consistently oppose any of the violence by all sides, but the responsibility is on the government to pull back its riot police, to call a truce and to engage in a meaningful discussion with the opposition about the way forward."

President Barack Obama was expected to comment on Ukraine later Wednesday during a trip to Toluca, Mexico, for a North American leaders summit, Rhodes said.

The close Obama advisor also warned of possible sanctions.

"We've made clear that we consider taking action against individuals who are responsible for acts of violence within Ukraine," he said.

"We have a toolkit for doing that that includes sanctions."

The Pentagon for its part called on Ukraine's army to stay out of the conflict and warned that if it intervened, Washington would review ties to the country's military.

"The Department of Defense is encouraged that the Ukrainian armed forces have not been brought into this crisis," spokesman Colonel Steven Warren told reporters.

"We urge them to remain on the sidelines. Participation would have consequences on our defense relationship," he said.

In a conversation with his Ukrainian counterpart in December, Defense Secretary Chuck Hagel urged the armed forces to exercise "restraint," Warren said.

The US government requested more than $100 million in aid for Ukraine for fiscal year 2013, including $1.9 million for military training and $7 million to finance the purchase of American-made weapons and equipment, according to the Congressional Research Service.

The overnight bloodshed came after some three months of mostly peaceful protests, which broke out after Yanukovych ditched a pact with the European Union in favor of closer ties with former Soviet master Russia.

On Tuesday, US Vice President Joe Biden called Yanukovych to express Washington's "grave concern" regarding the crisis, saying government forces that stormed a protest camp on Kiev's Independence Square, or Maidan, should withdraw.

The White House said Biden made clear in the call that the United States condemned violence by any side, "but that the government bears special responsibility to de-escalate the situation."

US lawmakers also voiced dismay at the violence.

Republican Senator Marco Rubio, a potential 2016 presidential candidate, urged the Obama administration "to use every diplomatic means at its disposal, including sanctions, to bring accountability to those involved in acts of violence throughout Ukraine."

Senate Democrat Chuck Schumer called the violence "unacceptable," adding on Twitter that "the Ukrainian people deserve freedom of speech and assembly."

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