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.
No shots were fired in the confrontation Wednesday, officials said, but the United States renewed a vow that it would protect its forces in the region.
The Pentagon initially said one of the US aircraft discharged a flare as a warning to the Iranian plane but officials later said no flare was let off.
The incident, which the Pentagon said took place over "international waters," highlighted the tensions between the two arch-foes and the risks of an accidental clash escalating into a serious crisis.
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, spokesman George Little said.
The unarmed Predator, the workhorse of America's fleet of robotic planes, was carrying out "a routine classified surveillance flight" over the Gulf when it was approached by the Iranian warplane, he said in a statement.
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 warned it would keep up 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, deploying minesweepers and F-22 fighters to the area.
This came 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 that a misunderstanding or accident involving the two countries could snowball into 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.
In an interview with Israel's Channel 2 television released Thursday, US President Barack Obama repeated his warning that all options remained "on the table" should diplomacy fail to prevent Iran from obtaining nuclear weapons.
Obama also said it would take "over a year or so" for Tehran to develop an atomic weapon but that "we don't want to cut it too close."