At least 328 people were killed and over 2,500 injured by a quake in Iran's Kermanshah province. on the Iraqi border, the Students News agency ISNA reported on Monday.
ISNA quoted the coroner's office in the province as saying that the death toll was expected to increase.
The quake hit 30 kilometers (19 miles) southwest of Halabja in Iraqi Kurdistan at around 9.20 pm, when many people would have been at home, the U.S. Geological Survey said.
Iran's emergency services chief Pir Hossein Koolivand said it was "difficult to send rescue teams to the villages because the roads have been cut off... there have been landslides."
The worst-hit towns in Iran were Qasr-e Shirin in Kermanshah and Azgaleh, about 40 kilometers northwest, IRNA said.
It added that 30 Red Cross teams had been sent to the quake zone, parts of which had experienced power cuts.
In Iraq, officials said the quake had killed six people in Sulaimaniyah province and injured around 150.
In the Iraqi province of Sulaimaniyah, residents ran out onto the streets and some damage to property was reported, an AFP reporter there said.
It was also felt in southeastern Turkey, "from Malatya to Van", an AFP correspondent said. In the town of Diyarbakir, residents were reported to have fled their homes.
The quake struck along a 1,500 kilometer fault line between the Arabian and Eurasian tectonic plates, a belt extending through western Iran and into northeastern Iraq.
The area sees frequent seismic activity.
A catastrophic quake that struck Bam, in Iran, in 2003 killed at least 31,000 people and flattened the city.
Since then, Iran has experienced at least two major quake disasters, one in 2005 that killed more than 600 and another in 2012 that left some 300 dead.
More recently, a 5.7-magnitude earthquake near Iran's border with Turkmenistan in May killed two people, injured hundreds and caused widespread damage