A Nato official said there was no indication a U.S. reconnaissance drone had been shot down in Iran, despite reports to the contrary from the country's media.
Military forces had shot down an unmanned spy plane, according to the country's semi-official Fars news agency.
The Iranian report could refer to an unarmed U.S. spy drone that went missing in Afghanistan last week, according to Nato.
The Arabic-language Al Alam state television network quoted an unnamed military source, who said: 'Iran's military has downed an intruding RQ-170 American drone in eastern Iran.'
The military insider added that Iran's response to the downed U.S. drone's violation of its airspace will not be limited to the country's borders.
A report on English-language Press TV said the drone was 'downed with minimum damage' and seized by Iranian officials.
Nato's International Security Assistance Force in Afghanistan said in a statement: 'The UAV (unmanned aerial vehicle) to which the Iranians are referring may be a U.S. 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.'
A U.S. official added: 'There is absolutely no indication up to this point that Iranians shot down this drone.'
The RQ-170 Sentinel is an unmanned stealth aircraft used for reconnaissance. It is not designed to carry weapons.
There was no immediate comment on the claims from the U.S. military.
The reports came amid worsening relations between the West and Iran over its nuclear programme after the UN's International Atomic Energy Agency (IAEA) issued a report it said showed Iran had worked on designing an atomic bomb.
Read it all