At least four people were killed when a powerful gas explosion destroyed part of a five-storey apartment block in Russia's Yaroslavl city early Tuesday morning, officials said."Unfortunately, we have found four bodies and four others have been hospitalised," said Yaroslavl regional governor Sergey Yastrebov.At least 39 people were reported trapped in the rubble, said sources from the Russian emergency situations ministry, EFE news reported.All those killed in the blast, which occurred at 4.20 a.m., are women.A staircase in the building collapsed after the explosion tore through 15 apartments, completely destroying five homes, Yastrebov said.Preliminary investigations have revealed that the explosion was caused by a gas leak, said law enforcement officials.The explosion did not trigger any big fire, but authorities cautioned that they could not rule out the possibility of further collapse of the building. The ministry also warned of more casualties.