TEHRAN - Three explosions have been heard northeast of Iran's central Isfahan province, the semi-official Fars news agency reported on Friday.

The cause of the explosions has not yet been determined, Fars quoted local sources as saying, adding that the army's radar had been among the possible targets and some buildings' windows in the nearby area had been broken.

According to Fars, the explosions were heard in Qahjavarestan city, near Isfahan's International Airport and an Iranian air force base.

The Iranian capital Tehran is now in good order, a Xinhua correspondent observed.

Meanwhile, the official IRNA news agency reported that Iran activated its air defense system over several cities on Friday.

Restrictions imposed on Friday morning on the operation of a number of airports in several Iranian cities including Tehran, Isfahan and Shiraz have been lifted, IRNA said.

Three micro aerial vehicles (MAVs) were spotted near Zardanjan village in Isfahan at around 4 am local time (0030 GMT) and were shot down by Iran's air defense, according to the semi-official Mehr news agency.

"There was no damage to Iran's nuclear sites" in an attack in northeast of Iran's central Isfahan province, the International Atomic Energy Agency (IAEA) confirmed Friday.