TEHRAN, Iran — A strong earthquake in southeastern Iran killed at least 20,000 people in the historic city of Bam early Friday, according to Iranian officials who appealed for international assistance in searching for survivors and recovering the dead.

The devastating 6.7 magnitude quake struck at 5:28 a.m. local time (5:58 p.m. PST Thursday), an hour that found almost all of the city’s 80,000 residents in their beds on the Muslim day of rest. Officials who surveyed the ancient Silk Road oasis by air estimated that 60 percent of dwellings collapsed, killing thousands almost instantly and injuring as many as 30,000 people, according to Iranian state television. Four thousand of the injured had been flown by helicopter to hospitals outside the affected area, state television said.

The deputy governor of Kerman province said an accurate count was impossible with many victims still trapped under the rubble. "Rescue operations are going slowly because of darkness," deputy governor Mohammad Farshad said.

But the Interior Ministry later released a statement saying the early estimate was 20,000 people — about a quarter of the Bam’s population.

Video images from Bam, 630 miles southeast of the capital city of Tehran, showed a vista of desolation that appeared to extend for miles. On residential streets lined with the city’s trademark eucalyptus trees, whole blocks of homes had collapsed onto their square lots, loose bricks spilling over sidewalks where bodies lay neatly tied in fuzzy blankets.

Loved ones squatted beside the corpses, weeping and brushing dust from the faces of the dead. A wailing man cradled the body of a child in one hand, and held his head with the other.

Bam, a starkly beautiful city founded 1,800 years ago, has been a favored tourist site, known for its massive mud ramparts and citadel with 38 towers, most of which were built starting in the 16th century. Long a center for commerce, Bam is located on the old Silk Road trade route through Asia, and was considered a world cultural site of major importance.

An aerial view of Bam on Friday showed much of the city’s sprawling modern section all but indistinguishable from its historic quarter. Witnesses said the city’s medieval fortress, renowned as the largest mud brick structure in the world, was reduced to rubble by the quake. Experts say the energy released in a 6.7 earthquake is roughly equal to a one megaton hydrogen bomb.