" Welcome to Incirlik AB, Turkey," reads the big gold letters over one of the entrances to a key staging base for the fight against the Islamic State, strategically located in southern Turkey, just 70 miles from the Syrian border.

But this month, Germany learned that the friendly greeting is not written in stone, and that the NATO ally can pull in its welcome mat with little notice or explanation.

After the government of Turkish President Recep Tayyip Erdogan refused to guarantee that members of the German parliament could visit German troops at Incirlik, Berlin announced it was moving its forces and planes to Jordan.

Germany's decision to decamp to more friendly territory starkly illustrates the growing tensions between Turkey and some of its NATO allies, including the United States.

"When our major European ally pulls its forces out of Incirlik because it couldn't be guaranteed access, it should warn you what happened to Germany could happen to us," said Joe Cirincione, president of the Ploughshares Fund, a Washington think tank focused on nuclear weapons policy.

"I don't think many people would count Erdogan as one of our more reliable allies. His agenda has shifted so dramatically over the last few years, you have to be concerned where it's going," Cirincione said.

While the U.S. continues to operate out of Incirlik, it too has had contentious relations with Ankara over a long list of issues, including America's backing for Kurdish fighters in Syria who Turkey regards as terrorists, and refusal to extradite U.S.-based Turkish cleric Fethullah Gulen, who Erdogan blames for last July's failed coup.

And in January, Turkey' Defense Minister Fikri Isik made a veiled threat about access to Incirlik, after the U.S. withheld air support for a Turkish operation in Northern Syria, which the U.S. thought was more about stopping the Kurds than defeating ISIS.

Turkey's rocky relations with its NATO partners, and Erdogan's increasing tilt toward Russia and Iran, has arms control advocates asking, "Is this really where we want to keep dozens of the most powerful bombs on Earth?"

"I would call Turkey a doubtful ally," said Mark Fitzpatrick, with the International Institute for Strategic Studies. "Turkey has been moving farther and farther away from democratic principles that are part of the nature of NATO."

As a matter of policy, the United States does not confirm or deny the presence of nuclear weapons at specific locations, but it is an open secret among national security experts that Turkey is among the five NATO nations to have U.S. nukes on its soil.

U.S. European Command, which oversees U.S. operations at Incirlik Air Base, while not acknowledging the presence of nuclear weapons at the base, said there are no security concerns overall. "Our strategic assets are stored under highly secure conditions and under U.S. control," said Air Force Capt. Joe Alonso. "We are confident that they are safe."

Hans Kristensen of the Federation of American Scientists, considered the foremost authority on the subject, said the U.S. keeps about 50 B-61 nuclear gravity bombs at Turkey's Incirlik base, each with a maximum yield of 170 kilotons, or 10 times more powerful than the bomb dropped on Hiroshima.

All this just about an hour-long drive from Syria, the most active war zone in the world, where in addition to a raging civil war, the international coalition led by the U.S. is battling ISIS, a terrorist group that would love to get its hands on a nuclear weapon.

"We should be concerned, and it is certainly past time to think about relocating them," Fitzpatrick told the Washington Examiner. "The nuclear weapons in Turkey are not currently under direct threat of seizure, but the circumstances over the past year give strong reason to take them out as a precaution."

Arm control advocates such as Ploughshare's Cirincione said there are many arguments for pulling U.S. nuclear weapons out of Turkey before something goes wrong.

He points to when, during the attempted coup last year, senior Turkish officers were accused of being among the leaders of the coup and flying missions from Incirlik in support of it.

The United States temporarily lost access to the base, and then for several days Turkish forces loyal to Erdogan surrounded Incirlik and cut off power, which Cirincione said effectively trapped some 2,500 U.S. military personnel along with the 50 nuclear weapons.

A week later, he pointed out, the base was again under siege, surrounded this time by thousands of anti-American protesters who burned American flags and demanded the government close the base.

"When you have four or five indicators that are all blinking red, and no sound strategic rationale for keeping the weapons there, you should err on the side of security, you should pull them out," Cirincione said.

The U.S. continues to say publicly that Turkey remains a strong and vital ally.

"I've made nine trips to Turkey in the past 12 months," Joint Chiefs Chairman Gen. Joe Dunford said at a recent appearance at the National Press Club. "I've met with my Turkish counterpart probably no less than 15 times in the last year to try to make sure we maintain a very effective relationship with a NATO ally."

Kristensen, of the Federation of American Scientists, says commercial satellite imagery shows there have been significant security upgrades made at Incirlik in the past two years.

The U.S. B-61 bombs, he said, are stored in 21 underground vaults inside the new security perimeter, with two to three bombs in each vault.

Still, Kristensen wrote in a 2015 blog post, "The wisdom of deploying NATO's largest nuclear weapons stockpile in such a volatile region seems questionable."

But perhaps the strongest argument for moving the bombs to a more stable location is that there is no compelling strategic reason that they have to be in Turkey.

"It wouldn't undermine U.S. deterrence, particularly if the weapons were moved to another country," Fitzpatrick said. "Even taking them out and putting them in the United States wouldn't undermine the deterrence policy. The weapons are there mainly for symbolic purposes, and they still are in four other NATO countries."

Those countries are Belgium, Netherlands, Germany and Italy, according to the Federation of American Scientists.

As for U.S. - Turkey relations, "Turkey is a close NATO ally and a vital member of the counter-ISIL coalition," Alonso said. "The U.S. military has worked very closely with our Turkish allies for decades to counter a wide range of threats to our common security."

CORRECTION: An earlier version of this story stated that the military acknowledges the presence of nuclear weapons at Incirlik. As a matter of policy, the military does not confirm or deny the location of nuclear weapons.