After a delay of more than five years, Iran has begun operating its Bushehr nuclear reactor, the heads of the Iranian and Russian atomic agencies announced on Saturday.

The 1,000-megawatt facility is designed to generate electricity, and experts and military officials in Israel, the United States and Western Europe say the prospect the reactor will be used in Iran's military nuclear program is extremely small.

The head of Iran's atomic energy agency, Ali Akbar Salehi, held a news conference at Bushehr on Saturday with the head of Russia's Rosatom state nuclear power company, Sergei Kiriyenko. Salehi said a symbol of Iran's peaceful use of nuclear technology had been launched despite pressure by Western countries.

The nuclear fuel rods at Bushehr, supplied by Russia, are enriched to less than 3.5 percent. The insertion of the rods in the past several days put the reactor online, but the start-up process will last several weeks and the plant will only be fully operational next month.

The facility is under the close supervision of the Russians and the International Atomic Energy Agency. Russian advisers will inspect the reactor regularly to verify that it is not being used for purposes that contradict the agreement between the two countries.

The agreement provides that the Iranians must return the spent fuel rods to Russia, letting the Russians verify that the rods are not being improperly used or transferred for use in Iran's clandestine military nuclear program.

On Saturday, the United States sent a calming message to Israel stating that the new reactor was designed for peaceful purposes for producing electricity and does not threaten Israel.

The Iranians have said they need additional nuclear reactors to produce electricity to meet the country's population growth and development rate, particularly in light of its declining reserves of oil and gas. Theoretically, however, the reactor has the potential to provide another source of fissionable material that could be used for nuclear weapons.

Currently the main concern about the country's nuclear weapons program is the enrichment activity at Natanz and perhaps at a secret unknown location.