WASHINGTON, Feb 13: The US has been flying surveillance drones over Iran for nearly a year to seek evidence of nuclear weapons programs, The Washington Post reported on Sunday.

Besides looking for nuclear sites, the US spy planes are also trying to detect weaknesses in Iran's air defence, the report added. The small, pilot less planes enter Iranian airspace from US military facilities in Iraq.

They use radar, video, still photography and air filters designed to pick up traces of nuclear activity to gather information that is not accessible by satellites, the report said.

US, European and Iranian officials told the Post that the Iranian government is aware of this surveillance and has formally protested the incursions as illegal. Since Iran does not have diplomatic relations with the US, the Iranian government used Swiss channels for conveying their protest.