US fighter jet shoots down armed Turkish drone over Syria

A U.S. fighter jet shot down a Turkish drone armed with air-to-surface missiles that was flying extensively in the area of U.S. ground troops in northeast Syria on Thursday, the Pentagon confirmed.

An F-16 shot the drone down after a dozen unheeded calls to Turkish military officials stating American forces were in the area and the U.S. would engage in self-defense if it didn't leave, a U.S. official had told ABC News.

"This is certainly a regrettable incident," Defense Department press secretary Brig. Gen. Pat Ryder said. "It's regrettable when you have two NATO allies involved in an incident like this."

Defense Secretary Lloyd Austin called his Turkish counterpart earlier Thursday to talk about the importance of strong communication between the U.S. and Turkish militaries, Ryder said. Gen. Charles Q. Brown Jr., the new chairman of the Joint Chiefs of Staff, also called his Turkish counterpart, according to Brown's spokesman.

Ryder said the incident began when U.S. aircraft spotted an armed Turkish drone conducting airstrikes in an area that the U.S. has declared as a "restricted operating zone" (ROZ), where U.S. troops were about a kilometer from the targeted area. Those U.S. troops went into bunkers for their safety, though Ryder said it didn't appear the Turkish aircraft was deliberately targeting U.S. troops.

Four hours later, a Turkish drone once again entered the ROZ and came within half a kilometer of U.S. forces, he said. After warnings were given and the drone continued above, U.S. commanders on the ground took "quick action" and determined that U.S. aircraft could take self-defense action, Ryder said.

A U.S. official told ABC News that the first incident involved multiple Turkish drones that carried out multiple airstrikes.

The shootdown of the Turkish drone comes days after Turkey conducted retaliatory airstrikes in northern Iraq against locations related to the outlawed Kurdish Workers Party (PKK), the militant Kurdish group that claimed responsibility for recent bomb attacks in Ankara.

Reuters reported Thursday that the Turkish Defense Ministry said in a statement that the drone did not belong to the Turkish armed forces.

Both the United States and Turkey are members of the NATO alliance, and Thursday's shootdown will likely increase the tensions between the two countries, which had previously been at odds over Turkey's refusal to allow Sweden to join the alliance.

There are about 900 U.S. troops based in eastern Syria who continue to work with Syrian Kurdish forces to prevent a resurgence by the Islamic State. Known as the Syrian Democratic Forces (SDF), Turkey has repeatedly claimed the SDF and the PKK are the same organization.

The U.S. has previously criticized Turkish airstrikes inside Syria that targeted Kurdish forces operating in close proximity to American forces.

Last November, U.S. officials said one Turkish airstrike came within 130 meters of American military personnel.

"These strikes put our troops at risk. They also risk the continuation of the defeat ISIS mission," a U.S. Central Command spokesman said at the time.

That airstrike came at a time when the Turkish military was conducting a ground operation against SDF forces in northern Syria that led the Kurdish group to suspend its military operations with the U.S. military.