US officials have voiced concern in recent months over increased Russian air patrols around Japan and South Korea and as far as the California coast and the US territory of Guam.
The State Department said Thursday it is aware of an agreement allowing Russia to operate Il-78 refueling planes out of Cam Ranh Bay, a US base during the Vietnam War.
The department said the US respects Vietnam's right to enter into agreements with other countries, but has urged Vietnam to ensure that Russia doesn't use its access "to conduct activities that could raise tensions in the region."