At least seven people were killed in a natural gas explosion in an apartment building in the Russian city of Yaroslavl, the country's emergency ministry said.Seven bodies have been recovered in the debris in the search and rescue operation, Xinhua quoted the ministry as saying on Tuesday.At least eight people were hospitalised and 131 inhabitants had been evacuated for fear of further collapse of the building.The explosion took place in a five-story residential building in Yaroslavl, about 250 km of Moscow, destroying five flats and damaging 20 others, the ministry said.Over 600 people and 150 units of equipment were dispatched for the search and rescue operation, including Mi-8 and Ka-32 helicopters.The statement did not clarify how many people were at home in the collapsed block at the moment of the explosion.Rescuers were still working to remove the rubble, and psychologists of the ministry continued providing assistance to victims, their relatives and others, the ministry said.Governor of Yaroslavl Region Sergei Yastrebov declared Wednesday and Thursday two days of mourning for the victims of the accident in the city.