An Iranian fighter jet tried to intercept a US Predator drone over the Gulf but backed off after encountering two American military aircraft, the Pentagon said Thursday.
"One of the US aircraft discharged a flare as a warning to the Iranian plane, which then broke off pursuit," spokesman George Little said in a statement.
No shots were fired in the confrontation on Wednesday, which the Pentagon said took place over "international waters."
At one point the Iranian F-4, an old US-built warplane dating from the Vietnam War era, was within 16 miles (25 kilometers) of the unmanned Predator drone, the statement said.
In November, an Iranian fighter jet fired at a Predator plane, provoking a strongly-worded protest from the United States.
As after the November incident, the Pentagon vowed to continue to conduct surveillance flights over what it deems international waters and to safeguard US forces in the region.
Little said that "we reserve the right to protect our military assets as well as our forces and will continue to do so going forward."
In December 2011, the Iranians captured a sophisticated Sentinel spy drone after it crashed on Iranian territory, in an embarrassment for Washington.
The United States expanded its military presence around the Gulf over the past year after Iran threatened to block the strategic Strait of Hormuz in retaliation for tough international sanctions imposed over its nuclear program.
US officials and military commanders worry about an accidental clash in the area that could escalate into a serious crisis and conflict.
But they are also keen to maintain a robust American military role in the region to counter Iran and to monitor its naval deployments.
President Barack Obama has repeatedly warned that he has not ruled out possible military action to prevent Iran from obtaining nuclear weapons.