ABOARD THE TCG ALEMDAR (Reuters) - In blazing sunlight, two dozen U.S. and Turkish sailors on a NATO exercise lower an American diving bell from an advanced Turkish rescue ship, sending it deep under the Aegean Sea where it is secured to a submarine.     Part of a combined NATO rescue simulation this week off Turkey s southwest coast, the seamless cooperation at sea comes amidst a storm between Ankara and its alliance allies who are  concerned about its decades-old commitment to the organization. Under President Tayyip Erdogan, Turkey, with NATO s second-biggest army, has sought to bolster ties with Russia and Iran.  In a clear sign of rapprochement, Ankara is buying a missile defense system from Russia - unnerving NATO officials, who are already wary of Moscow s military presence in the Middle East, as the system is incompatible with the alliance s systems.   Turkey said it opted for the S-400 anti-aircraft system because Western arms suppliers had not offered a  financially effective  alternative. The Pentagon said it expressed concern to Turkey about the deal.  They went crazy because we made the S-400 agreement. What were we supposed to do, wait for (them)?  Erdogan said recently.  If we have difficulty in obtaining any defense element from some places, if our initiatives are often frustrated by obstacles, what will we do? We will sort it out on our own.  Erdogan s frustration stems from Washington s support for the Syrian Kurdish YPG in the fight against Islamic State. Ankara sees the YPG as an extension of the outlawed Kurdistan Workers Party (PKK), which has carried out an insurgency in Turkey s largely Kurdish southeast and is considered a terrorist organization by the United States and Europe as well as Turkey. The president was also angry that U.S. prosecutors charged his former economy minister for conspiring to violate U.S. sanctions on Iran. The indictment marked the first time an ex-government member with close ties to Erdogan had been charged in the investigation that has strained relations between Washington and Ankara.  Part of the reason Erdogan is doing this S-400 deal is he s angry with the U.S. over the indictment of the former economy minister as well as continued U.S. cooperation with the YPG,  said Soner Cagaptay, a fellow at the Washington Institute think-tank and author of  The New Sultan: Erdogan and the Crisis of Modern Turkey .  He s using the S-400 as a lever, in terms of bargaining, to convince Washington to change its mind on a number of issues.  Ties with Europe, especially Germany, were hit by Turkey s crackdown after last year s failed coup. Some 150,000 people were purged from the civil service, military and private sector and over 50,000 jailed, including German nationals. Alarmed by what it sees as Ankara s deteriorating record on human rights, Germany has said it would restrict some arms sales to Turkey. It initially announced a freeze on major arms sales, but scaled that back, citing the fight against Islamic State. Ankara also refused to allow German lawmakers to visit their troops stationed at an air base in Turkey. This has led Germany to move troops involved in the campaign against Islamic State from Turkey s Incirlik base to Jordan. Turkey rejects the idea it is turning away from the West.  The good relations Turkey has developed with Russia are not an alternative to the good relations it has with the West, they complement each other,  Erdogan spokesman Ibrahim Kalin said. Erdogan told Reuters in an interview on Thursday that Turkey s position in NATO had not been weakened by the deal. Still, some fear Turkey might eventually find itself at the periphery of the alliance.   Germany is our most important supplier of weapons after the United States,  said Umit Pamir, a former Turkish diplomat. He said the suspension of arms sales, would  surely impact our defense systems . Erdogan has been on a push to improve ties with Moscow after Turkey s economy, particularly its tourism industry, was shaken by sanctions imposed by Russia after Turkey shot down one of its warplanes over Syria in late 2015. He is due to meet Russian President Vladimir Putin next week to discuss a plan agreed by their countries and Iran to reduce the fighting in Syria s northwestern Idlib province. Turkey supports rebels against Syrian President Bashar al-Assad, who is backed by Russia and Iran.  Erdogan plans to visit Iran next month. The two countries agreed in August to boost military cooperation when Iran s military chief, General Mohammad Baqeri, met Erdogan in a visit. The trip was the first by an Iranian military chief of staff to Turkey since the 1979 Islamic revolution in Iran. While they back opposite sides in Syria, Ankara and Tehran have found some common ground over their opposition to the Kurdish independence referendum in Iraq. Both countries fear that an independent Kurdish state could inflame separatists tensions with their Kurdish minorities. Recent discord over the S-400 purchase did not signal a drastic break from NATO for Turkey, said Mustafa Kibaroglu, a professor of international relations at Turkey s MEF University. He said the West had over-reacted to the purchase.   I don t think there is any debate about Turkey leaving an alliance it has invested so much in,  Kibaroglu said.  Are we going to bring down U.S. planes with our S-400s?  he said.  There is no backbone to these comments, it is purely political polemics, and we are not the ones doing this.  