An explosion ripped apart a small bus in a separatist Moldovan region Thursday, killing eight people and injuring 46, state media said.

The blast in the Trans-Dniester capital Tiraspol blew the roof and sides off the bus, which had 12 passengers. Eight were killed and four badly injured, the private news agency Lenta PNR said.

Most of those injured were passers-by, and 10 were passengers on another bus who were hurt by the impact of the blast, Lenta PNR reported, citing Interior Ministry investigators. The windows of that bus were blown out.

The cause of the explosion was not immediately known. But Oleg Beleakov, the separatists' deputy interior minister, was quoted as telling RIA Novosti that a passenger might have been carrying a bomb meant for a commando killing involving criminal business groups, and it might have exploded accidentally on the bus.

More than half of those injured were seriously hurt, prosecutor Ivan Lesukov was quoted as saying by Moldova's official Olvia Pres news agency.

Trans-Dniester broke away from Moldova with Russian support in 1992 after a war that left more than 1,500 people dead. An uneasy peace has dominated the region since then, and there have not been outbreaks of violence.

The province is not recognized internationally. Russia maintains about 1,500 troops in the region along the Ukrainian border, which it considers to be strategically important.

In Washington, the State Department condemned those responsible for the bus explosion and said the U.S. was ready to assist authorities investigating it.

Moldovan President Vladimir Voronin offered assistance to Trans-Dniester police to find the perpetrators and medical aid to victims.

Among the injured in Thursday's blast were two Russian peacekeepers, Beleakov was quoted as saying.

Moldova's relations with Moscow have soured in recent years over the Kremlin's support for the separatists, and it has moved to establish closer ties to the West.

The European Union and the Organization for Security and Cooperation in Europe have said they were concerned that the Trans-Dniester border was a conduit for illegal goods, drugs, stolen cars and illegal immigrants making their way into Ukraine and, in some cases, on to the EU.

There have been isolated incidents involving weapons in Trans-Dniester, but nothing as serious as Thursday's explosion.