Turkey is a member of NATO and an aspiring member of the European Union -- but it has one alliance that sets it apart from its Western counterparts: It's an important base of operations for at least one high-ranking member of the Palestinian militant group Hamas. Turkish Prime Minister Recep Tayyip Erdogan has made no secret of his desire to transform Hamas into an accepted member of the international community. In 2011, he told a U.S. audience that the Palestinian party was not a terrorist group, and he has repeatedly vowed to visit the Hamas-controlled Gaza Strip. Ankara has also provided Hamas with significant financial support -- as much as $300 million, according to some estimates. In his attempts to strengthen Hamas, Erdogan has also allowed his country's ties with Israel to suffer. The Turkish leader famously stormed offstage during a contentious 2009 panel with Israeli President Shimon Peres, in protest of Israel's isolation of Gaza. Relations between Ankara and Jerusalem plummeted further the following year, after Turkey's largest NGO dispatched a flotilla that tried to break Israel's blockade of Gaza, leading to clashes between Israeli commandos and activists that left nine Turks dead. More recently, however, the two countries have take steps to bury the hatchet. This year, U.S. President Barack Obama facilitated a phone call between Erdogan and Israeli Prime Minister Benjamin Netanyahu, which began a process that resulted in Israel issuing an apology for the incident and agreeing to pay reparations to the victims' families. Mutual interests in Turkey -- namely the ouster of Syria's Bashar al-Assad -- have provided additional hope for rapprochement. However, Erdogan's support for Hamas could become a serious stumbling block for a further warming of ties with Israel. The Turkish premier's ties with Hamas remain as strong as ever -- in fact, they appear to have deepened. Turkey currently serves as the home for Hamas operative Saleh al-Arouri, whom the Palestinian movement's website identifies as the founder of the Izz al-Din al-Qassam Brigades, Hamas's armed wing, in the West Bank. One senior Israeli intelligence official described him to me as "one of the most important leaders of Hamas ... involved in a lot of things including finance and logistics." Arouri's presence in Turkey raises the stakes in what the official calls a "dirty game" that Ankara is playing with the militant group. Just this year, Hamas's military wing in the West Bank attempted to kidnap soldiers and civilians and even planned to bomb an outdoor shopping mall. As the head of the West Bank's Qassam Brigades, Arouri may well have directed those attacks from Turkey. Arouri was originally recruited by Hamas while studying at Hebron University, and he has served as a high-ranking military leader for the movement since the early 1990s, according to U.S. court documents. After serving several stretches of jail time, Israel released him in March 2010, possibly as part of an effort to secure the release of captured Israeli soldier Gilad Shalit. After Arouri's release, he served as a political official in Hamas's headquarters in Damascus, where he reportedly played a role in negotiating the Shalit deal, which brokered the soldier's freedom for more than 1,000 Palestinians in Israeli custody. When Hamas parted ways with Syria over the Assad regime's massacres in the country's ongoing civil war, Arouri left Damascus and is believed to have started operating out of Turkey last year. He has not been shy about his presence there: In March 2012, for example, he was part of a Hamas delegation that took part in talks with Turkish officials, including Erdogan. In October 2012, he traveled from Turkey to Gaza to attend the visit of Qatar's emir to the Hamas-controlled Gaza Strip. But diplomacy appears to be only one part of Arouri's job. He is also allegedly involved in Hamas's illicit financial networks. In April 2013, Israeli security services announced the arrest of two Palestinians for smuggling money from Jordan to Hamas operatives in the West Bank. During the interrogation, according to the Shin Bet, Israel's internal security service, one smuggler admitted that he was moving the money upon the orders of Arouri. Presumably, those orders were issued from Turkey. The veteran Israeli analyst of Palestinian affairs, Ehud Yaari, recently noted that Turkey is allowing Arouri to direct efforts to rebuild Hamas's terrorism infrastructure in the West Bank. If Arouri really has, as Yaari writes, "taken sole control of the movement's activities in the West Bank," Turkey appears to have in effect taken over from Damascus and become Hamas's West Bank headquarters. On a recent trip to Turkey, two parliamentarians from the ruling Justice and Development Party (AKP) told me they had no knowledge of Arouri's presence or activities. Similarly, two senior officials from the Turkish IHH charity -- which sponsored the 2010 pro-Hamas flotilla to Gaza and which the United States believes has provided Hamas with material assistance -- said they did not know Arouri's name. Even Western diplomats claimed ignorance of his whereabouts. Given the strategic importance of Turkey to the United States, particularly in light of Turkey's role in helping to support the Syrian opposition, officials in Washington have demurred on confronting Ankara. Obama, who has maintained cordial ties with Erdogan, has given no indication that Turkey's relationship with Hamas is a problem for Washington. The only notable exception was a bipartisan congressional letter in May that expressed "concerns about Turkey's relationship with Hamas." But a recent uptick in Hamas terrorism out of the West Bank may change Washington's calculus. Israel's Shin Bet recently foiled a Hamas plot to establish a terrorist cell in the West Bank city of Hebron. Meanwhile, there have been seven attempted attacks out of the West Bank so far this year, compared with six all last year. If Arouri is behind the funding, recruiting, or planning of any of these Hamas operations in the West Bank, it will have grave consequences for Turkey. To the letter of the law, Turkey could meet criteria as a state sponsor of terrorism. Strange friends for a nation that views itself part of the Western alliance.