ROSTOV-ON-DON, Russia (AP) _ Two bombs exploded under a commuter train Wednesday in southern Russia, killing at least four people and wounding dozens of others, officials said.

The bombs were planted on the tracks linking Kislovodsk to Mineralnye Vody in the Caucasus region. There were about 50 people in the third car of the six-car train, which was directly hit by the blast, Railway Ministry spokesman Konstantin Pashkov said.

There were varying reports on the casualty toll.

Dmitry Oliferenko, a spokesman for President Vladimir Putin's envoy to southern Russia, said five people were killed, while Russian Railways Minister Gennady Fadeyev said in televised comments that six died _ three in the explosions and three others afterward.

But regional Emergency Situations Ministry spokesman Igor Mikhailov said four were killed and 33 were wounded in the explosions as the train was approaching a station in Podkumok, a small town on the outskirts of Kislovodsk, 870 miles south of Moscow.

Nine of the injured were in grave condition and six others were in serious condition, Kislovodsk health care department chief Viktor Chuprikov said on NTV television.

No group immediately claimed responsibility for the blasts.

Viktor Kazantsev, President Vladimir Putin's envoy to southern Russia, told state television that police had arrested a man suspected of detonating the bombs.

Russia has been rocked recently by numerous bombings and other attacks, which the government usually blames on Chechen rebels.

An officer at the headquarters of the Caucasus Military District, which oversees Chechnya, said on condition he not be identified that the military had received intelligence information that the rebels were preparing a series of attacks in southern Russia.

The explosion occurred just as Putin was scheduled to chair a meeting of regional governors in Rostov-on-Don, about 280 miles northwest of the site of the explosions. Putin spoke with Stavropol governor Alexander Chernogorov about the blasts, news agencies reported.