Obama, EU's Tusk stress unity in imposing sanctions on Russia over Ukraine crisis

U.S. President Barack Obama and European Council President Donald Tusk on Monday underlined the importance of maintaining trans-Atlantic unity to impose costs on Russia over the Ukraine crisis.

"We are all committed to making sure that we uphold the basic principles of sovereignty and territorial integrity that have been threatened by Russian aggression," Obama told reporters at the White House before meeting with Tusk.

"We've been able to maintain strong unity with respect to sanctions (against Russia)," Obama said, hailing trans-Atlantic unity that is "as strong as it's ever been."

Obama said he and Tusk would discuss how to effectively monitor the situation in Ukraine, and how to continue to maintain pressure on Russia and the rebels in eastern Ukraine to abide by the ceasefire agreements reached last month.

Tusk said a need for unity of the Europe and the U.S. is the " common denominator" in addressing the challenges the two sides face.

"We can see with full clarity" that the enemies "are trying to divide us," Tusk said. "But when we are united we will be able to put a stop to the aggressive policy of Russia against her neighbors."

"We have to not only speak in one voice but also to act in unison," the former Polish Prime Minister added.

The U.S. and Europe have been largely in sync in responding to the Ukraine crisis, slapping several rounds of economic sanctions against Russia over its annexation of Crimea and "destabilizing moves" in eastern Ukraine. But the Obama administration had said providing lethal weapons to Kiev is one of the options, a move some European countries firmly oppose.

Before heading to Washington, D.C., Tusk acknowledged in an interview published by the New York Times that the European Union' s complex decision-making made it difficult for the bloc to respond as swiftly and firmly to Russia and other security challenges as Washington would like.

Besides the Ukraine crisis, Obama and Tusk will also discuss, among other things, the threat from the Islamic State extremist group and the negotiations on Transatlantic Trade and Investment Partnership. Endite