Iran's Fars news agency on Tuesday reported that Iranian fighter jets have forced a US aircraft to land in Iran after it entered its air space without permission, but the report was denied by the Pentagon.

"The plane had five soldiers and three civilians on board who were subjected to questioning," Fars said.

"After a day's interrogation, it emerged that the aircraft did not enter Iran intentionally and it was allowed to leave for Afghanistan."

State television said the aircraft carrying the soldiers was a non-American civilian plane.

"This plane was not a military plane and did not belong to the United States," an unnamed official was quoted as saying by the website of the television's Arabic-language Al-Alam channel.

"But a few US military soldiers were in the plane," he official added.

The plane was forced to land in Iran on Sunday and allowed to fly to Afghanistan on Monday, the website added.

A spokeswoman for the US Fifth Fleet Lieutenant Stephanie Murdock told AFP by telephone from its base in Bahrain she had "no information right now" about the Iranian reports.

A Pentagon spokesman said none of its aircraft were missing.

"According to the combined air operations centre, all our aircraft are accounted for and we have no reports of any aircraft landing in Iran," said Lieutenant Colonel Patrick Ryder.

In April 2007, Iran released 15 British sailors and marines whom it detained for almost two weeks saying their vessels had strayed into its waters on the maritime border with Iraq.