WASHINGTON: US President Barack Obama and Vice President Joe Biden have expressed concerns about fresh tensions between Russia and Georgia in telephone calls with the leaders of the rival ex-Soviet states.

The White House said Russian President Dmitry Medvedev called Obama to wish him a happy 48th birthday on Tuesday, and that the leaders discussed the need to calm rattled nerves in the region, a year on from a Russia-Georgia war.

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 US officials concerned about the potential for rising tensions between Moscow and Washington.

It was not clear whether the movements of the submarines, first reported by the New York Times on its website late Tuesday, 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, ahead of the anniversary of their conflict over rebel South Ossetia.

"Russian President Medvedev called President Obama today to wish him happy birthday," the White House said in a press statement.

"During the call, the presidents discussed the situation in Georgia and the need to decrease tensions in the region.

"President Obama reiterated the importance of working through established crisis management mechanisms such as the Joint Incident Prevention and Response Mechanism and underscored the need for international monitors."

Later, the White House said Biden called Saakashvili to "discuss the current situation in Georgia."

"Vice President Biden expressed concern about the recent escalation in tensions and emphasized that all parties should avoid destabilizing actions."

Biden also "reiterated US support for Georgia's democracy."

Saakashvili had earlier called on the United States and the European Union 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.

The Russian foreign ministry meanwhile said its forces had heightened their state of battle-readiness in South Ossetia.

"The situation is very worrying and the Georgian provocations ahead of the anniversary of last year's war are not halting," foreign ministry spokesman Andrei Nesterenko said in a statement.

Obama's administration is walking a tightrope between its desire to reset ties with Moscow and showing support for its ally Georgia, as tensions rise again between Moscow and Tbilisi.

Russia smashed a Georgian military offensive to recapture South Ossetia in a brief war in August last year, sending relations between Moscow and Washington during the final months of George W. Bush's administration to post-Cold War lows.

Biden risked irking Russia last month when he said in a speech in the ex-Soviet republic that Obama backed Georgia's aspiration to join NATO.

The White House also said that Obama and Medvedev used Tuesday's conversation to discuss the need to "move forward quickly" on agreements reached at their summit last month in Moscow.

"In particular, the presidents reaffirmed their commitment to complete negotiations on a follow-on agreement to START by December of this year."

Obama and Medvedev signed a declaration in Moscow pledging to reach a new nuclear arms pact to replace the 1991 Strategic Arms Reduction Treaty.

But the latest submarine revelations threaten to raise tensions once again.

Citing defense and intelligence officials speaking on condition of anonymity, the Times reported that one of the submarines remained in international waters some 200 miles (320 kilometers) off the US coast, while the location of the other remained unclear.

"Any time the Russian Navy does something so out of the ordinary it is cause for worry," a senior US Defense Department official who has monitored reports about the submarines was quoted as saying.