MOSCOW - As many as 40 people could still be trapped in the rubble of a Russian apartment block that partially collapsed in an explosion on Monday, killing at least four people, news agencies reported.

The blast, thought to have been caused by a gas leak, damaged 48 apartments in a nine-storey building in Magnitogorsk, an industrial city in the Urals some 1,700 km (1,050 miles) east of Moscow, the emergencies ministry said.