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.