A US official has denied Iranian claims that an American drone was shot down in the country’s east, while Nato suggested the unmanned aircraft merely crashed.
Earlier in Tehran, state TV quoted a military source as saying a US reconnaissance aircraft had been shot down along the border.
The unmanned spy plane was violating Iran’s airspace, it was claimed.
The military source warned that the country’s armed response would “not be limited to our country’s borders” for the “blatant territorial violation”.
An American official, who declined to be named, said: “There is absolutely no indication up to this point that Iranians shot down this drone.”
The Nato-led force in Afghanistan said the aircraft may be an unarmed surveillance drone that was lost last week while flying over western Afghanistan.
A statement from the International Security Assistance Force (Isaf) added the drone “may” belong to the US.
“The operators of the UAV (unmanned aerial vehicle) lost control of the aircraft and had been working to determine its status,” it said.
There are around 140,000 foreign troops in Afghanistan, about 100,000 from the US, fighting a Taliban-led insurgency.
The latest report comes at a time of heightened political tension over Iran’s nuclear programme, with speculation rife that Israel is mulling air strikes against Iranian atomic facilities, with or without US backing.
The news also comes as relations between Pakistan and the US hit a new low after 24 Pakistani soldiers were killed in an air strike near the Afghan-Pakistani border last month.
In July, US officials dismissed a report that a US drone had been downed over the city of Qom, close to the Fordu nuclear facility.
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.
Iran has insisted its nuclear ambitions are entirely peaceful, but prompted the US and European Union to agree tighter sanctions on Iranian companies and individuals.
Earlier on Sunday, Iran’s foreign ministry warned that any move to extend those sanctions to block oil exports would double the price of crude.
Britain has also downgraded its relations with Iran following the storming of the UK embassy in Tehran by students protesting over the sanctions on Iran’s central bank.
British diplomats in Tehran were evacuated following the incident and Iranian embassy staff in London were expelled.