The United States has urged Vietnam to stop letting Russia refuel military planes at a key Vietnamese base because of concerns it could raise tensions in the Asia-Pacific, the US State Department said on Thursday.

The move could put some strain on growing ties between Hanoi and Washington. US officials have said in recent months that Russia has increased air patrols around Asia and is operating as far as the California coast and US territory of Guam.

The US State Department did say that it is aware of an agreement allowing Russia to operate the Ilyushin Il-78 refueling planes out of Cam Ranh Bay, a US base during the Vietnam War.

But the department also said the US respects Vietnam's right to enter into agreements with other countries. Spokeswoman Jen Psaki urged Vietnamese officials to ensure that Russia does not use its access "to conduct activities that can raise tensions in the region".

A spokesman at Vietnam's embassy in Washington did not immediately respond to a request for comment.

Vietnam has sought a closer relationship with the US. In October, the Obama administration partially lifted a ban on lethal arms sales to Vietnam, and last week, the US ambassador to Hanoi said that the chief of the ruling communist party, Nguyen Phu Trong, will visit Washington this year.

The deep sea port at Cam Ranh Bay served as a major Soviet naval base after the Vietnam War, and Russia maintained a presence there until 2002. Vietnam subsequently upgraded the base, offering it as a site for maintenance of foreign naval ships.