Just hours after the calls, reports emerged that Russian nuclear-powered submarines were patrolling the waters off the US East Coast, the first such move in several years and one that has American officials concerned about the potential for rising tensions between Moscow and Washington.

It was not clear whether the movements of the submarines were discussed by Medvedev and Obama. Biden called Georgian President Mikheil Saakashvili and expressed concern over the Russia-Georgia situation, as Georgia warned of the risk of a new war with Russia and Moscow raised the battle-readiness of its forces.

“Russian President Medvedev called President Obama today to wish him happy birthday,” the White House said. “During the call, the presidents discussed the situation in Georgia and the need to decrease tensions in the region.”

Saakashvili had earlier called on the US and the EU to send a “clear message” to Moscow to help avert a new war, as both sides exchanged accusations of attacks and “provocations” in the region.