Iran said Wednesday that unidentified foreign planes violated its airspace six times as the country kicked off its biggest ever air defense drill but that the intruders were intercepted and forced back by Iranian jets.

The remarks by Gen. Hamid Arjangi, a spokesman for the exercise, were the first Iranian claim of an intrusion. Initially, he had only said that foreign reconnaissance planes had approached Iran's air space.

There was no way to verify Iran's claims. The spokesman did not specify whether the aircraft were warplanes or pilotless surveillance drones that might have been sent up to monitor the drill.

A spokesman for the U.S. Navy's 5th Fleet, based just across the Persian Gulf in the island kingdom of Bahrain, said he had no information on the purported airspace intrusion.

Arjangi said Iran's radar stations and observation posts picked up on the planes entering Iranian airspace during the five-day drill, which started Tuesday.

"There were six cases of intrusion by unidentified planes into the country," Arjangi was quoted as saying by the official IRNA news agency. "In all six cases, air force jet fighters took off and carried out interception operations ... artillery systems were alerted, targets were identified and necessary warnings were given."

The Iranian exercise is meant to showcase the country's capabilities in defending its nuclear facilities from possible attack.

It followed an announcement by the Iranian Air Force saying its troops earlier this year conducted an exercise at several facilities — the uranium enrichment plant at Natanz, the under-construction enrichment site at Fordo, the nuclear conversion facility near Isfahan and the Bushehr nuclear power plant.

Arjangi said thousands of surveillance outposts have been stationed along 4,400 miles (7,000 kilometers) of Iran's border and are equipped with sophisticated communication systems capable of countering enemy jamming to transfer data to control command centers.

He did not specify whether the stations were on the section of border along the Persian Gulf.

Gen. Ahmad Mighani, head of an air force branch in charge of responding to threats to Iran's airspace, said Tuesday the war games seek to "upgrade the combat preparedness" of the country's air defense system.

The United States and its European allies accuse Iran of embarking on a nuclear weapons program. Iran denies the charge and insists the program is only for peaceful purposes.

Israel has not ruled out military action to stop Iran from obtaining nuclear weapons. Iran, in turn, has said Israel would face a "devastating retaliation" if it attacked the Islamic Republic.

Iran is expected to unveil a locally made radar system with a range of some 1,800 miles (2,900 kilometers) during the drill.

Iran's armed forces conduct several war games every year and frequently unveil new weapons and military systems during those exercises.

Iran restructured its military last year in an effort to improve its air defenses. The changes were part of a broader focus on bolstering the military amid concerns over the U.S. military's presence in neighboring Iraq and Afghanistan.

Iran's Supreme Leader Ayatollah Ali Khamenei ordered a new branch to be split off from the air force last year to deal exclusively with threats to the country's airspace. Mighani was appointed to head the branch.