The death toll from an earthquake in southern Iran has climbed to at least 25,000 people, Agence France-Presse reported, citing a medical official.

The dead include 5,000 already buried and 20,000 still under the rubble, the report said, citing Iraj Sharifi, an official at the medical faculty in Kerman, north of the quake-devastated city of Bam.

A senior government official told Reuters, "The (death toll) is now more than 20,000." Survivors lit fires to stay warm in the open amid the mass of flattened mud-brick houses. Survivors using their bare hands joined search teams tearing at rubble.

The 6.5 magnitude quake struck Bam at 5:26 a.m. Friday local time, according to the U.S. Geological Survey. It was followed by at least one strong aftershock. Among structures that collapsed were two of the city's hospitals, crushing many employees, the official IRNA news service reported, citing Kerman provincial governor general Mohammad-Ali Karimi. Karimi said 60 percent of the city's buildings were destroyed. About 200,000 people live in and around the city.

The Iranian Red Crescent sent rescue teams by helicopter to the region and a transport plane to ferry some of the injured to hospitals in other parts of the country, IRNA said. Communications were cut with two other towns in the area.

The United States pledged to offer humanitarian assistance. Russia and the United Nations have also offered aid. Japan will send an emergency rescue team and equipment worth $230,000 to assist relief operations, the Kyodo news agency reported.

The team will consist of a doctor, a nurse, two Japan International Cooperation Agency workers and a Foreign Ministry official, Kyodo reported, citing government officials it didn't identify. The equipment will be sent within a few days, the report said.

The Silk Road city of Bam, consisting of houses made of mud bricks and the trunks of palm trees, has a fortified citadel constructed 2,000 years ago and other buildings and monuments dating back to the 12th century.

Bam was an important trading post for dates and textiles before falling to Afghan invaders in 1719. Most residents left the ancient walled city in the late 1950s, and it became a museum.

The historic part of the city was surrounded by walls 12 meters high (39.4 feet) and a moat. Almost all of the historic city was destroyed by the earthquake, IRNA said. Pictures broadcast on Iranian state television showed extensive damage throughout the city.

The earthquake is the deadliest since 2001, when more than 20,000 people were killed in India by a 7.7 magnitude earthquake that also injured 166,000 and left 600,000 homeless, according to the U.S. Geological Survey.

Earlier this week, a 6.5 magnitude earthquake struck the central California coast. Two people were killed when a building collapsed in Paso Robles, including a former Utah woman.

Although the earthquake was felt as far away as San Francisco and Los Angeles, damage was limited because that part of California is thinly populated.