"As he was being interviewed, the man blew himself up with an unidentified explosive device," Astakhov said.

An unidentified man blew himself up on the Ukrainian border with Russia early Friday, seriously wounding several guards who spotted he was carrying a false passport, Ukraine's border service said.

Border guards stopped a minibus with Moldovan numberplates as it crossed into Ukraine from Russia through the Bachevsk border, an advisor to the head of Ukrainian border guards, Sergiy Astakhov, told AFP.

The guards found a fifth passenger hidden inside the minibus, who then showed a Russian passport "which did not belong to him," Astakhov said.

"As he was being interviewed, the man blew himself up with an unidentified explosive device," Astakhov said.

"The man himself was killed, while two Ukrainian border guards have been hospitalised with serious injuries.

The motive for the bombing, which took place shortly after midnight (9PM GMT) was not immediately clear.

The border crossing point has been closed while an investigation continues and the whole Russian-Ukrainian border has been put on high alert, Russian media reported.

The head of Ukraine's border guards, Mykola Lytviyn, was at the scene on Friday morning.