Authorities in Burundi are preventing public transport vehicles from crossing the border into Rwanda, in an escalation of tensions over allegations that Rwanda backs rebels opposed to Burundi's government.

The operators of some passenger buses transporting people between the two countries say they have been stopped by Burundian border officials who ask passengers to leave the vehicles and cross the border on foot.

Pierre Nkurikiye, a spokesman for Burundian police, says the government wants to stop all food exports from Burundi, which faces a possible food crisis amid political violence following the president's decision last year to seek a third term.

Burundi's government accuses Rwandan authorities of training and arming rebels with the aim of removing President Pierre Nkurunziza, allegations Rwanda denies.