Russian President Vladimir Putin has called US President Barack Obama to discuss international negotiations over Iran�s nuclear program ahead of a self-imposed deadline for a final agreement.

The two leaders underscored the importance of �unity� as negotiators from Iran and the P5+1 group of countries, which includes the US and Russia, are meeting in Vienna to hammer out the final details of a comprehensive deal by June 30, the White House said on Thursday.

The United States is concerned that Russia may wield its veto power at the United Nations Security Council if a need emerges for re-imposing sanctions on Iran in the wake of a nuclear deal.

�We will not support a snap-back mechanism or an agreement that includes a snap-back mechanism that leaves us vulnerable,� Samantha Power, the US ambassador to the UN, said during a June 16 hearing at the House Foreign Affairs Committee.

�We will retain the ability to snap back multilateral sanctions architecture back in place without Russian or Chinese support,� she stated.

US Secretary of State John Kerry is travelling to the Austrian capital city of Vienna on Friday for a final push to seal a historic agreement with Tehran.

According to the White House statement, Obama and Putin also discussed the Ukrainian conflict, which has plunged US-Russia relations to their lowest level since the Cold War.

�President Obama reiterated the need for Russia to fulfill its commitments under the Minsk agreements, including the removal of all Russian troops and equipment from Ukrainian territory,� the White House statement said.

Washington accuses Moscow of arming and supporting pro-Russia forces in the predominantly Russian-speaking areas in eastern Ukraine in their fight against the pro-Western central government in Kiev. The Kremlin calls the accusations "groundless."

The US and its NATO allies in Eastern Europe have stepped up their military moves, including exercises and the creation of a NATO rapid response force.

The military buildup in NATO member states bordering Russia has drawn strong objections from Moscow, followed by warnings of a well-measured response.

Three weeks ago, Obama met with other leaders of the G7 industrial nations at a summit in Germany, pressing them to reinforce their efforts to isolate Putin.

�Does he (Putin) continue to wreck his country�s economy and continue Russia�s isolation in pursuit of a wrongheaded desire to recreate the glories of the Soviet empire?� Obama said at the close of the summit.

By Press TV