Washington (AFP) - Dozens of US nuclear weapons stored at a Turkish air base near Syria are at risk of being captured by "terrorists or other hostile forces," a Washington think tank claimed Monday.

Critics have long been alarmed by America's estimated stockpile of about 50 nuclear bombs at Incirlik in southern Turkey, just 70 miles (110 kilometers) from the border with war-torn Syria.

The issue took on fresh urgency last month following the attempted coup in Turkey, in which the base's Turkish commander was arrested on suspicion of complicity in the plot.

"Whether the US could have maintained control of the weapons in the event of a protracted civil conflict in Turkey is an unanswerable question," said Monday's report from the Stimson Center, a nonpartisan think tank working to promote peace.

Incirlik is a vital base for the US-led coalition fighting the Islamic State group in Iraq and Syria, with the strategically located facility affording drones and warplanes fast access to IS targets.

But the Pentagon in March ordered families of US troops and civilian personnel stationed in southern Turkey to quit the region due to security fears.

"From a security point of view, it's a roll of the dice to continue to have approximately 50 of America's nuclear weapons stationed at Incirlik Air Base in Turkey," report co-author Laicie Heeley said.

"There are significant safeguards in place. ... But safeguards are just that, they don't eliminate risk. In the event of a coup, we can't say for certain that we would have been able to maintain control," she told AFP.

- 'Avoided disaster so far' -

While the Pentagon does not discuss where it stores nuclear assets, the bombs are believed to be kept at Incirlik as a deterrent to Russia and to demonstrate America's commitment to NATO, the 28-member military alliance that includes Turkey.

The Incirlik nuke issue has been the subject of renewed debate in the United States since the coup attempt.

"While we've avoided disaster so far, we have ample evidence that the security of US nuclear weapons stored in Turkey can change literally overnight," Steve Andreasen, director for defense policy and arms control on the White House National Security Council staff from 1993 to 2001, wrote in an opinion piece in the Los Angeles Times last week.

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Kori Schake, a fellow at the California-based Hoover Institution, noted in a written debate in the New York Times that "American nuclear forces cannot be used without codes, making the weapons impossible to set off without authorization."

"The fact that nuclear weapons are stationed in Turkey does not make them vulnerable to capture and use, even if the country were to turn hostile to the United States," she argued.

The Pentagon declined to comment on questions arising from the Stimson study.

"We do not discuss the location of strategic assets. The (Department of Defense) has taken appropriate steps to maintain the safety and security of our personnel, their families, and our facilities, and we will continue to do so," it said in a statement.

The Incirlik concerns were highlighted as part of a broader paper into the Pentagon's nuclear modernization program, through which the United States would spend hundreds of billions of dollars to update its atomic arsenal.

The authors argue that a particular type of bomb -- the B61 gravity bomb -- should be immediately removed from Europe, where 180 of the weapons are kept in Belgium, Italy, Germany, the Netherlands and Turkey.

The report can be viewed at: http://u.afp.com/ZV9i