Washington: The United States has asked Vietnam to stop letting Russia use a former US base to refuel nuclear-capable bombers engaged in shows of strength over the Asia-Pacific region, exposing strains in Washington’s steadily warming relations with Hanoi.

The request, described to Reuters by a State Department official, comes as US officials say Russian bombers have stepped up flights in a region already rife with tensions between China, US-ally Japan and Southeast Asian nations.

General Vincent Brooks, commander of the US Army in the Pacific, told Reuters the planes had conducted ‘provocative’ flights, including around the US Pacific Ocean territory of Guam, home to a major American air base.

It is the first time that US officials have confirmed the role of Cam Ranh Bay, a natural deep-water harbor, in Russian bomber plane activity that has increased globally.

Brooks said the planes that circled Guam were refueled by Russian tankers flying from the strategic bay, which was transformed by the Americans during the Vietnam War into a massive air and naval base.

Vietnam’s willingness to allow Russia to use Cam Ranh Bay reflects Hanoi’s complex position in a geopolitical tug-of-war that frequently pits China and Russia on one side and the United States, Japan and much of Southeast Asia on the other.

Washington is keen to secure greater access itself to Cam Ranh Bay as part of its strategic ‘pivot’ to Asia to counter China’s growing strength in the region. US ships have visited for repairs in recent years.

Vietnam, in turn, has sought closer US ties as a hedge against what it sees as China’s aggression, but remains close to Russia in both defence and energy cooperation.

Cam Ranh Bay is now host to three submarines bought by Vietnam’s navy from Russia to counter Chinese expansion in the South China Sea, with two more expected by early next year.

Brooks said in an interview the flights indicated that Vietnam’s Cold War-era ally Russia was acting as ‘a spoiler to our interests and the interests of others.’

Raising Tensions

Asked about the Russian flights in the region, the State Department official, who spoke on condition of anonymity, said Washington respected Hanoi’s right to enter agreements with other countries.

But the official added: ‘We have urged Vietnamese officials to ensure that Russia is not able to use its access to Cam Ranh Bay to conduct activities that could raise tensions in the region.’

The Vietnamese government did not immediately respond to a request for comment on the US request.

Brooks declined to say when the flights he referred to took place. He did not say how many had been conducted and how many aircraft were involved. But he confirmed they had occurred since Russia’s annexation of Crimea last March, which sparked a broader conflict with Ukraine and a surge in tensions between Russia and the United States.

The head of US air forces in the Pacific said last May that Russia’s intervention in Ukraine had been accompanied by a significant increase in Russian air activity in the Asia-Pacific region in a show of strength and to gather intelligence.

Russia’s Defence Ministry said on 4 January that Russian Il-78 tanker aircraft had used Cam Ranh Bay in 2014, enabling the refuelling of nuclear-capable TU-95 ‘Bear’ strategic bombers, a statement also reported in Vietnam’s state-controlled media.

In that time, Russia has conducted increasingly aggressive air and sea patrols close to the borders of the US-led NATO alliance, including by Bear bombers over the English Channel.

Last year, NATO conducted more than 100 intercepts of Russian aircraft, about three times as many as in 2013.