By David Brunnstrom

WASHINGTON (Reuters) - The United States has asked Vietnam to stop letting Russia use a former U.S. 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 U.S. officials say Russian bombers have stepped up flights in a region already rife with tensions between China, U.S.-ally Japan and Southeast Asian nations.

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

It is the first time that U.S. 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. U.S. ships have visited for repairs in recent years.

Vietnam, in turn, has sought closer U.S. ties as a hedge against what it sees as China's aggression, but remains close to Russia in both defense 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."

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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 U.S. 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 U.S. 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 Defense Ministry said on Jan. 4 that Russian Il-78 tanker aircraft had used Cam Ranh Bay in 2014, enabling the refueling 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 U.S.-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.

RUSSIAN BOMBER FLIGHTS

Russian bomber patrol flights, a Cold War-era practice, were cut back after the fall of the Soviet Union but President Vladimir Putin revived them in 2007.

Russia said in November it planned to send long-range bombers on patrols over North American waters but the Pentagon played this down at the time as routine training in international airspace.

In its effort to boost ties with Vietnam, the United States has been pouring in aid and assistance in health, education, landmines clearance, scholarships and nuclear energy.

Defense cooperation had been limited by an embargo on lethal arms. But Washington started to ease this in October, enabling humanitarian exercises between both militaries late last year and more are taking place this month.

Last year saw a flurry of high-level U.S. visits to Vietnam that coincided with a maritime territorial row between Hanoi and Beijing. On Friday, the U.S ambassador in Vietnam announced that the Vietnamese Communist Party chief would later this year become the first party leader to visit Washington.

U.S. Ambassador to Vietnam Ted Osius said on Friday it was understandable Hanoi would look to "historic partners" when it came to security, but the United States had "much to offer... to enhance Vietnam's security in the short, medium and long term."

(Reporting by David Brunnstrom; Additional reporting by Martin Petty and Ho Binh Minh in Hanoi and Jason Szep in Washington; Editing by David Storey and Stuart Grudgings)