More than 300 people were killed in Iran when a magnitude 7.3 earthquake jolted the country on Sunday, state media said, and rescuers were searching for dozens trapped under rubble in the mountainous area.
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 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.
A magnitude 4.7 earthquake struck Monday about 23 miles east of Salinas, sending shaking waves felt as far away as San Francisco.
The epicenter of the earthquake, which struck at 11:31 a.m. at a depth of 3.7 miles, was centered in the sparsely populated mountains between the Salinas Valley and the San Joaquin Valley.
The U.S. Geological Survey calculated that the perceived shaking close to the epicenter was strong, or Intensity 6, on the Modified Mercalli Intensity Scale. Weak shaking was reported felt in Salinas, Monterey and Santa Cruz, and all the way north to some parts of the Bay Area, including San Francisco, Foster City and San Jose.
In the last 10 days, there have been no earthquakes of magnitude 3.0 or greater centered nearby.