There were no immediate reports of deaths from the quake, which was measured at a magnitude of 6.2 and was felt across a broad swath of western Iran and eastern Iraq, officials said. At least 80 people were injured, according to Iranian news reports.
The quake struck at a relatively shallow depth of about six miles, with an epicenter close to the small town of Mourmouri, 15 miles from the city of Abdanan in the eastern Ilam province. The site is about 300 miles southwest of Tehran.
The stricken area is close to an active fault zone that often experiences moderate seismic events, authorities say.
Monday's earthquake followed two weaker temblors in the region on Sunday that prompted warnings for people to sleep outdoors. Tents were distributed to area residents as a precaution, Iranian media reported.
The advice to stay outdoors likely saved lives as many buildings were damaged or collapsed, authorities said. Rescue teams, some in helicopters, were headed to the stricken zone.
Much of the damage may have been in remote, mountainous areas, authorities said. The quake cut water and electricity service across a wide swath of territory, prompting officials to rush water supplies to people in the intense summer heat.
"We are still feeling several aftershocks," said Khalil Rashnavi, a writer in the neighboring Khuzestan province who was reached three hours after the earthquake.
Authorities said they feared the casualty toll would likely rise once rescue workers reached the affected areas across the Iranian provinces of Ilam, Lorestan and Khuzestan.
Mostaghim is a special correspondent. Times staff writer Patrick J. McDonnell in Irbil, Iraq, contributed to this report.