Vice President Joe Biden during a visit to Ukraine warned Russia to follow through on a diplomatic deal and take steps to de-escalate tensions between the two countries.

“It’s time for Russia to stop talking and start acting,” Biden said in Kiev, alongside Ukrainian Prime Minister Arseniy Yatsenyuk.

The vice president called on Moscow to pull its troops back from the border and for pro-Russian groups to return government buildings seized in the eastern part of the country back to Ukrainian control.

“We need to see these … steps and we need to see them without delay,” said Biden. “We will not allow this to be an open-ended process.”

The vice president is in Ukraine to reassure the interim government of U.S. support as pro-Russian militants in the east of the country call for autonomy and as Russian forces remain massed along the border.

A diplomatic deal between the two countries, the U.S. and European Union has failed to decrease tensions with Russian separatists saying they are not bound by the accord and Moscow accusing Kiev of not honoring the deal.

The Obama administration, though, has placed the blame for the crisis squarely on the Kremlin and warned of further sanctions if Russia does not take steps to defuse the situation.

Biden in Kiev called on Russia to "stop supporting men in masks in unmarked uniforms."

The vice president also said the U.S. was committed to helping Ukraine, as the White House announced an additional $50 million package to help Kiev implement political and economic reforms.

Biden said that Ukraine “must remain one country.”

“No nation has the right to simply grab land from another nation, no nation has that right,” he said, vowing that the U.S. would “never recognize Russia's illegal occupation of Crimea and neither will the world.”