An Iranian warplane opened fire on an unarmed U.S. military drone conducting surveillance near Iranian airspace Nov. 1, the Pentagon said Thursday, the first such incident over the Persian Gulf and one that is all but certain to draw attention to Washington’s use of unmanned aircraft.

The MQ-1 Predator drone returned to its base unscathed, even as the Iranian aircraft chased it away from the Islamic Republic’s borders, Pentagon spokesman George Little said Thursday, disclosing details of an incident that the Obama administration chose to keep quiet during the final stretch of the presidential campaign.

The encounter comes nearly a year after a U.S. drone crashed in eastern Iran, marking the first time that the United States acknowledged that one of its stealth unmanned aircraft fell into enemy hands. The Iranian government said its forces shot down the RQ-170 drone, but officials in Washington said it may have simply crashed.

The Nov. 1 incident happened at 4:50 a.m. Eastern time, approximately 16 miles from the Iranian coastline, Little said. Under international law, national sovereignty extends for 12 nautical miles.

An Iranian Su-25 fighter jet pursued the U.S. drone as it retreated from Iranian airspace, the spokesman said. The American aircraft does not appear to have been struck and landed safely at an unidentified base in the region.

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Little declined to say whether the engagement constituted an act of war.

“This is the first time a UAV has been fired upon by an Iranian aircraft,” he said, using the abbreviation for unmanned aerial vehicle. “The United States has communicated to the Iranians that we will continue to do surveillance flights over international waters over the Arabian Gulf.”

The United States conveyed the message through Swiss diplomats, who have represented U.S. interests in Tehran since Washington broke off diplomatic relations.

The United States has come to rely heavily on drones to spy on nations where it has limited human intelligence assets. In Iran, U.S. drones have been deployed to assess the country’s nuclear program. Nations at odds with Washington have grown wary of the United States’s growing drone fleet, which includes several armed models that are used routinely to strike suspected terrorist hideouts.

Iranian media had not reported on the Nov. 1 incident as of Thursday afternoon, in contrast to their boastful portrayal of the December 2011 crash of the U.S. drone near the country’s border with Afghanistan.