Iran has declared a three-day mourning period following a major earthquake that killed at least 37 people and injured 850 more in the country's southwest, according to state media reports.

Rescuers wound up operations on Wednesday near the port city of Bushehr, where Iran's only nuclear power plant is located.

"With the rescue operation being wrapped up, no one is left under the rubble," the Fars news agency quoted the head of Iran's Red Crescent rescue team, Mahmoud Mozafar, as saying.

Fars reported that some 700 homes had been destroyed.

Mozafar said efforts were now focused on relief operations including around 1,000 tents that had been set up in quake-hit areas.

Fars said blankets and food have been sent to stricken areas.

No radioactive release

Iran said it informed the International Atomic Energy Agency that there has been no damage to the nuclear power plant in Bushehr, nearly 90km northwest of quake's epicenter, and no radioactive release.

Iran's atomic chief Fereydoon Abbasi Davani said that the power plant was not operational when the quake struck as it was " under maintenance", Iranian media reported.

The Russian company that built the nuclear power station, 18km south of Bushehr, also said the plant was unaffected.

"The earthquake in no way affected the normal situation at the reactor. Personnel continue to work in the normal regime and radiation levels are fully within the norm," Russian state news agency RIA quoted an official at Atomstroyexport as saying.

Water and electricity were cut to many residents, said Ebrahim Darvishi, governor of the worst-hit district Shonbeh.

Shahpour Rostami, the deputy governor of Bushehr province, told state TV that rescue teams have been deployed to Shonbeh.

Three helicopters were sent to survey the damaged area before sunset, said Mohammad Mozaffar, the head of Iran's Red Crescent rescue department.

Mondani Hosseini, a resident of Kaki, told the Associated Press news agency that people had run out into the streets out of fear.

The quake was felt across the Gulf in Bahrain, the United Arab Emirates and Qatar, where workers were evacuated from highrise buildings as a precaution.

Rescue teams dispatched

The quake hit at 11:52 GMT with a depth of 12km, the Iran Seismological Centre said.

The US Geological Survey, which monitors quakes worldwide, ranked the quake at a more powerful 6.3 magnitude.

Iranian media said search and rescue teams had been despatched to the area, to which telephone connection has been cut.

The seismological centre has so far reported six aftershocks, the strongest at 5.3 magnitude.

Iran sits astride several major fault lines and is prone to frequent earthquakes, some of which have been devastating.

A double earthquake, one measuring 6.2 and the other 6.0, struck Tabriz, in the northwest, in August last year, killing more than 300 people and injuring 3,000 more.

In December 2003, a big earthquake struck the southern city of Bam. It killed 31,000 people - about a quarter of the population - and destroyed the city's ancient mud-built citadel.