A bus transporting workers after their shift at a local factory collided with a train Friday in southern Russia, killing at least 21 people, the Emergency Situations Ministry said.

Ministry spokesman Sergei Kozhemyaka said in Rostov-on-Don that all the victims were on the bus, which was carrying 27 people from the Kontsentrat factory. Six others were seriously injured, officials said.

The accident occurred at an unguarded train crossing near the village of Voronezhskaya under heavy fog and the engine car derailed, said another ministry spokesman, Viktor Beltsov. The bus was severely damaged, complicating rescue efforts.

The train was traveling from the resort city of Mineralnye Vody to Krasnodar, about 750 miles south of Moscow.

The regional governor's office declared Saturday and Sunday as officials days of mourning in the Krasnodar Territory, the Interfax news agency reported.

Russian news agencies initially reported that 17 schoolchildren were killed, but Kozhemyaka said the victims were all being transported from the factory.