MOSCOW — At least seven people died and dozens were missing on Monday after an early-morning explosion, possibly ignited by a gas leak, caused part of a 10-story residential apartment building to collapse in the central Russian city of Magnitogorsk, local news agencies reported.

Cries for help could be heard from the rubble, news reports said, with hundreds of rescue workers scrambling to find survivors. Witnesses smelled gas in the area, and a leak was believed to be behind the explosion in the industrial city at the southern edge of the Ural Mountains, some 1,050 miles east of Moscow.

The head of the Emergencies Ministry, Yevgeny Zinichev, said workers had found seven bodies, the TASS news agency reported, and the local government of Magnitogorsk said as many as 45 people were missing and could be trapped. The authorities were racing to find remaining survivors because cold weather was complicating a relief effort that would be difficult even under normal circumstances.

The temperatures, already frigid during the day at 0 degrees (minus 18 degrees Celsius), were expected to drop to about minus 17 at night, leaving anyone trapped vulnerable to exposure. There were also concerns that other parts of the building might collapse, a spokesman for the Emergencies Ministry said.