The Bush administration has been flying surveillance drones over Iran for nearly a year to seek evidence of nuclear weapons programs and detect weaknesses in air defenses, according to three U.S. officials with detailed knowledge of the secret effort.

The small, pilotless planes, penetrating Iranian airspace from U.S. military facilities in Iraq, 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 officials said. The aerial espionage is standard in military preparations for an eventual air attack and is also employed as a tool for intimidation.

Secretary of State Condoleezza Rice said the United States, concerned Iran is covertly developing nuclear arms, has no plans to attack "at this time." (Geert Vanden Wijngaert -- AP) _____Free E-mail Newsletters_____ • Today's Headlines & Columnists See a Sample  |  Sign Up Now • Breaking News Alerts See a Sample  |  Sign Up Now

The Iranian government, using Swiss channels in the absence of diplomatic relations with Washington, formally protested the incursions as illegal, according to Iranian, European and U.S. officials, all speaking on the condition of anonymity because of the sensitivity of the matter.

A U.S. official acknowledged that drones were being used but said the Iranian complaint focused on aircraft overflights by the Pentagon. The United States, the official said, replied with a denial that manned U.S. aircraft had crossed Iran's borders. The drones were first spotted by dozens of Iranian civilians and set off a national newspaper frenzy in late December over whether the country was being visited by UFOs.

The surveillance has been conducted as the Bush administration sharpens its anti-Iran rhetoric and the U.S. intelligence community searches for information to support President Bush's assertion that Tehran is trying to build nuclear weapons.

The Washington Post reported Saturday that the intelligence community is conducting a broad review of its Iran assessments, including a new look at information about the country's nuclear program, according to administration officials and congressional sources. A similar review, called a National Intelligence Estimate, formed an important part of the administration's case for war against Iraq.

Bush's senior advisers, including Secretary of State Condoleezza Rice and Defense Secretary Donald H. Rumsfeld, said last week that a U.S. attack on Iran is not imminent but that the option remains available.

In late December, Iranians living along the Caspian Sea and on the Iraq border began reporting sightings of red flashes in the sky, streaks of green and blue and low, racing lights that disappeared moments after being spotted. The Iranian space agency was called in to investigate, astronomy experts were consulted, and an agreement was quickly signed with Russian officials eager to learn more about the phenomena.

But the mystery was laid to rest by Iranian air force commanders, some of whom were trained more than 25 years ago in the United States and are familiar with U.S. tactics. They identified the drones early last month, a senior Iranian official said, and Iran's National Security Council decided not to engage the pilotless aircraft.

That action is considered a major policy decision and reflects Iran's belief that an attack is unlikely anytime soon.