Vice President Joe Biden spoke with Russian Prime Minister Dmitri Medvedev on Monday and urged Moscow to withdraw military forces from Ukraine's Crimean peninsula.

“The Vice President urged Russia to pull back its forces, support the immediate deployment of international monitors to Ukraine, and begin a meaningful political dialogue with the Ukrainian government,” said the White House in a statement.

Biden's call was the latest effort by the Obama administration to calm tensions after Moscow sent military forces into neighboring Ukraine. The White House is facing increasing pressure from allies and lawmakers on Capitol Hill to take a tough stand against Russia and defuse the situation.

Biden over the weekend spoke with Ukrainian Prime Minister Arseniy Yatsenyuk and President Obama held a 90-minute call with Putin on Saturday.

Obama warned Putin publicly on Friday that there would be “costs” for Russia's military intervention. The White House said Obama told his Russian counterpart during their call Saturday to respect Ukraine's independence and withdraw his forces.

The president “expressed his deep concern over Russia’s clear violation of Ukrainian sovereignty and territorial integrity,” the White House said.

Obama has also been working the phones to rally U.S. allies to stand with him in opposition to Russia's military moves. The White House said the president spoke with German Chancellor Angela Merkel, British Prime Minister David Cameron and Polish President Bronislaw Komorowski in separate calls over the weekend.

Secretary of State John Kerry is also slated to visit Kiev on Tuesday to highlight support for the interim government.

The standoff over Crimea has quickly become the toughest foreign policy challenge of Obama’s administration.

The White House said the administration would stop its preparations for the upcoming G8 Summit to be held in Russia and threatened that further steps could be taken. But many international observers question if the U.S. has enough leverage to force Putin's hand.

Reports Monday, citing Russian state media, said that Moscow had given Ukraine an ultimatum to handover Crimea or face a “military storm.”

Moscow says it is protecting the interests of ethnic Russians in its neighbor after the government in Kiev removed former Ukrainian President Viktor Yanukovych, a close ally of Russia, from power. Yanukovych fled the capital last month after a brutal crackdown by his security forces failed to stop a protest movement opposed to his push for closer ties with Russia.

Ukraine is politically divided between a western half that favors closer ties with the European Union and an eastern half with a large Russian population.

Obama told Putin in his call Saturday that he understood his concern for ethnic Russians in Ukraine, but that Moscow should work with the international community to address those issues.