ABU DHABI, United Arab Emirates — David Petraeus, the four-star U.S. Army general and former CIA director who oversaw years of operations in Afghanistan and Iraq, has been direct in his criticism of Turkey, calling the NATO ally's recent behavior "deeply concerning." But he still believes Washington needs Ankara as an ally, and must put in the work to mend strained relations, Gen. Petraeus, who is currently chair of the KKR Global Institute, told CNBC on Thursday. Asked by CNBC's Hadley Gamble how he felt about the Turkish government's foreign policy of late, he replied, "It is deeply concerning and we have to be very forthright with our Turkish allies about that." "This is a sensitive time in the relationship not just between Turkey and the U.S. but Turkey and its NATO partners writ large," Petraeus said while attending the annual Abu Dhabi International Petroleum Exhibition & Conference (ADIPEC). "The introduction of a Russian air defense system that will preclude Turkey from receiving the F-35 is obviously not welcome." A major thorn in U.S.-Turkey relations has been Ankara's purchase of the Russian-made S-400 missile defense system, deemed an operational security risk if used alongside the American F-35 fighter jet. The supersonic jets — which can fly for hundreds of miles undetected by radar and of which 110 were slated for Turkey's military — were withheld from the Turks after President Recep Tayyip Erdogan made clear his forces would buy the S-400 against U.S. wishes. "I'm sure that some of the discussions that they've had have been to try and find a path forward where something could be done so that that system is not operating at the same time as the F-35, if it is allowed to be sold to Turkey now, having been halted. That could go forward," Petraeus said.

"Turkey is a very, very important country to NATO, it is very geo-strategically important in its positioning and a variety of different ways. It played an important role as a base for our aircraft and other assets in the region, so again I hope this can be resolved." Turkey joined NATO in 1953, three years after its formation, and now has the second-largest military in the 70-year old alliance after the U.S. Its massive Incirlik airbase houses an estimated 50 U.S. nuclear bombs and 5,000 American military personnel, and is a vital launch point for NATO operations in the Middle East. Recent reports reveal that the U.S. military is reviewing evacuating its tactical nuclear weapons stored there as a result of the growing tensions between the two countries. Alongside NATO concerns over Erdogan's warming ties with Russia's Vladimir Putin, U.S. lawmakers and security experts have widely condemned Turkey's invasion of Northeastern Syria and its offensive against U.S.-allied Kurds governing the area that Ankara views as terrorists. Lawmakers have put forth bipartisan sanctions bills against Erdogan's government, accusing Trump of giving the Turkish leader a free hand in massacring the Kurdish militias who proved vital in the fight against ISIS. Petraeus previously described President Donald Trump's decision to withdraw U.S. troops from the area and abandon the Kurdish fighters as a "betrayal" and a serious strategic mistake. "I agreed very strongly with Senator (Majority Leader Mitch) McConnell at the time who judged that this was a grave strategic mistake," Petraeus said. "It was equal parts abandoning the Kurds, handing a victory to Iran and Russia who are not our friends, rendering a large population essentially homeless, possibly refugees, a degree of ethnic displacement if not ethnic cleansing, and perhaps most importantly of all, taking our eye off the Islamic State." Petraeus stressed that the current U.S. mission is to achieve the "enduring defeat" of ISIS, not merely the destruction of the physical caliphate in Iraq and Syria, warning that some 20,000 to 25,000 fighters still remained in the region and that "we need to keep an eye on them."