Iran "strongly protests against such unlawful acts and emphasizes the necessity to observe the principles of international law concerning the sanctity of the sovereignty and territorial integrity of states," its Foreign Ministry said.

The Pentagon had no immediate comment to the protest, which came in letters to the U.S. government written months ago but only made public at the United Nations on Monday.

They described the crash of a Shadow 200 RQ-7 drone in Ilam Province last July 4, 36 miles (60 kms) from the border, and of a Hermes drone in the Khoram Abad area on Aug. 25, found 120 miles (200 kms) inside Iranian territory.

An investigation found that both of the unmanned aircraft were American, the Foreign Ministry said. Its letters were delivered to the Swiss Embassy in Tehran for transmission to U.S. authorities as Iran has no diplomatic ties with the United States.

U.S. President George W. Bush once branded Iran as a part of an "axis of evil" pursuing weapons of mass destruction along with Iraq and North Korea. His administration has accused Tehran for more than two years of pursuing nuclear weapons under the guise of a peaceful nuclear energy program.

Britain and the United States also suspect Tehran of smuggling arms to insurgents in Iraq.

Tehran has denied helping fuel the insurgency in Iraq and says it has every right to pursue atomic power as a domestic energy source.

But the International Atomic Energy Agency, the Vienna-based U.N. nuclear watchdog agency, has accused it of keeping parts of its nuclear program secret for years, in violation of the nuclear Non-Proliferation Treaty.

The European Union has joined the United States in insisting that Iran freeze all its sensitive nuclear activities but EU states France, Britain and Germany said on Monday they were studying a new Iranian offer of negotiations aimed at resolving the impasse.