- Iran has a number of nuclear sites, which are monitored by the global watchdog, the International Atomic Energy Agency (IAEA).
- The earthquake was felt in several western provinces of Iran, but the hardest hit province was Kermanshah, which is where the Arak heavy water reactor and production plant is situated (see map)
- The head of Iranian Red Crescent said more than 70,000 people were in need of emergency shelter.
At least six have died in Iraq as well.
State television said more than 336 people were killed in Iran and at least 3,950 were injured.
Local officials said the death toll would rise as search and rescue teams reached remote areas of Iran. The earthquake was felt in several western provinces of Iran, but the hardest hit province was Kermanshah, which is where the Arak heavy water reactor and production plant is situated.
The existence of a heavy-water facility near the town of Arak first emerged with the publication of satellite images by the US-based Institute for Science and International Security in December 2002. Spent fuel from a heavy-water reactor contains plutonium suitable for a nuclear bomb, according to the BBC.
More than 236 of the victims were in Sarpol-e Zahab county in Kermanshah province, about 15 km (10 miles) from the Iraq border.
Iranian state television said the quake had caused heavy damage in some villages where houses were made of earthen bricks.
Rescuers were laboring to find survivors trapped under collapsed buildings.
The quake also triggered landslides that hindered rescue efforts, officials told state television.
At least 14 provinces in Iran had been affected, Iranian media reported.
Iranian Supreme Leader Ayatollah Ali Khamenei offered his condolences on Monday, urging all government agencies to do all they could to help those affected.
The U.S. Geological Survey said the quake measured magnitude 7.3.
An Iraqi meteorology official put its magnitude at 6.5 with the epicenter in Penjwin in Sulaimaniyah province in the Kurdistan region, close to the main border crossing with Iran.
Kurdish health officials said at least six people were killed in Iraq and at least 68 injured.
The quake was felt as far south as Baghdad, where many residents rushed from their houses and tall buildings when tremors shook the Iraqi capital.
"I was sitting with my kids having dinner and suddenly the building was just dancing in the air," said Majida Ameer, who ran out of her building in the capital's Salihiya district with her three children.
"I thought at first that it was a huge bomb.
But then I heard everyone around me screaming: 'Earthquake!"' Similar scenes unfolded in Erbil, the capital of the Kurdistan Region, and across other cities in northern Iraq, close to the quake's epicenter. Electricity was cut off in several Iranian and Iraqi cities, and fears of aftershocks sent thousands of people in both countries out onto the streets and parks in cold weather.
The Iranian seismological center registered around 118 aftershocks and said more were expected.
The head of Iranian Red Crescent said more than 70,000 people were in need of emergency shelter. Hojjat Gharibian was one of the hundreds of homeless Iranian survivors, who was huddled against the cold with his family in Qasr-e Shirin.
An Iranian oil official said pipelines and refineries in the area remained intact.
Iran sits astride major fault lines and is prone to frequent tremors.
A magnitude 6.6 quake on Dec. 26, 2003, devastated the historic city of Bam, 1,000 km southeast of Tehran, killing about 31,000 people.