Some 25 apartment buildings and a student dormitory collapsed in the town of Ercis on the north shore of Lake Van, the Turkish Red Crescent said.
Television pictures from Van Province showed rescuers and members of the public climbing over massive piles of cinderblocks that had been a building before the earthquake hit. Ambulances and bulldozers were on the scene.
A seven-story building collapsed on Kazim Karabekir Street in the city of Van, and more buildings were reduced to rubble the village of Tabanli in Van Province, the Anatolian news agency said. It was unknown how many people were trapped.
"There are so many dead. Several buildings have collapsed. There is too much destruction," Zulfikar Arapoglu, the mayor of Ercis, told NTV television. "We need urgent aid. We need medics."
Serious damage and casualties were also reported in the district of Celebibag, near Ercis.
"We can hear voices from the collapsed buildings," he said. "We need urgent aid, we need medics," he added.
The last quake of that magnitude in Turkey -- a 7.2 tremor in Duzce in 1999 -- killed 894 people, the USGS reported. A 7.6 earthquake in Izmit, Turkey, killed more than 17,000 people the same year, according to the USGS.
Sunday's major quake hit at 1:41 p.m. local time and was followed by at least seven aftershocks, American and Turkish monitoring agencies reported.