"Iran's military has downed an intruding RQ-170 American drone in eastern Iran," Iran's Arabic-language Al Alam state television network quoted the unnamed source as saying.
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"The spy drone, which has been downed with little damage, was seized by the Iranian armed forces."
The source said Iran's response to the downed US drone's violation of its airspace will not be limited to the country's borders.
Digital image of RQ-170 drone (Archive photo: Truthdowser CC)
"The Iranian military's response to the American spy drone's violation of our airspace will not be limited to Iran's borders any more," Iran's Arabic language Al Alam television quoted the military source as saying, without giving details.
The International Security Assistance Force (ISAF) said on Sunday a surveillance drone flying over western Afghanistan had gone out of control late last week and may be the one Iran said it had shot down.
"The UAV to which the Iranians are referring may be a US unarmed reconnaissance aircraft that had been flying a mission over western Afghanistan late last week. The operators of the UAV lost control of the aircraft and had been working to determine its status," an ISAF statement said.
The statement about the unmanned aerial vehicle (UAV) was issued in Kabul and released to reporters covering an international conference on Afghanistan in the German city Bonn.
A US official said there was "absolutely no indication" up to this point that the drone that crashed in Iran was shot down.
Iran said in July it had shot down an unmanned US spy plane over the holy city of Qom, near its Fordu nuclear site.
Iran shot down the drone at a time when it is trying to contain foreign reaction to the storming of the British embassy in Tehran on Tuesday, shortly after London announced that it would impose sanctions on Iran's central bank in connection with Iran's controversial nuclear enrichment program.
Britain evacuated its diplomatic staff from Iran and expelled Iranian diplomats in London in retaliation, and several other EU members recalled their ambassadors from Tehran.
The attack dragged Iran's relations with Europe to a long-time low.
Washington and EU countries have been discussing measures to restrict Iran's oil exports since the United Nations nuclear watchdog issued a report in November with what it said was evidence that Tehran had worked on designing an atom bomb.
Iran says its nuclear program is entirely peaceful.