Iranian warplanes fired at an unarmed U.S. drone in international airspace last week but did not hit the aircraft, the Pentagon said on Thursday, an unprecedented incident that triggered a sharp warning to Tehran through diplomatic channels.

The Pentagon said the Nov. 1 intercept appeared to be the first time Tehran has fired at an unmanned American aircraft, in this case an MQ-1 “Predator.”

According to details provided by the Pentagon, two Iranian SU-25 “Frogfoot” aircraft intercepted the American drone at about 4:50 a.m. ET (0850 GMT) as it conducted a routine, but classified, surveillance mission about 16 nautical miles off the Iranian coast.

Little said the aircraft fired multiple rounds at the Predator drone and followed it for at least several miles as it moved farther away from Iranian airspace.

“We believe that they fired at least twice and made at least two passes,” he said.

International airspace begins after 12 nautical miles and Little stressed that at no point did the drone enter Iranian airspace.

The United States sent Iran a warning through diplomatic channels about the Nov. 1 intercept, saying it would defend its military assets and would keep sending aircraft on such surveillance operations.

“There is absolutely no precedence for this,” Little said. “This is the first time that a (drone) has been fired upon to our knowledge by Iranian aircraft.”

The incident, which follows Iran’s recovery of a crashed CIA drone in its territory last year, was another reminder of how tensions between the United States and Iran could quickly escalate as the West tightens pressure on Tehran over its nuclear program.