President Laurent Kabila's troops have forced 500 Hutu refugees back to Burundi from the Democratic Republic of Congo, the U.N. refugee agency UNHCR said Wednesday.

Spokesman Kris Janowski said UNHCR was concerned by the expulsions, which began Monday in the eastern town of Uvira. Troops commandeered aid agency trucks for the expulsions and also marched some refugees to the border, he told a briefing.The group of Hutus from Burundi and Rwanda also included local Congolese apparently expelled by mistake.

Relations between UNHCR and Kinshasa have been strained since agency staff were expelled from the eastern town of Goma last year and work for Rwandan Hutu refugees was suspended.

Kabila's government has blocked UNHCR access to some 173,000 Rwandan Hutu refugees who remain unaccounted for.

Aid officials believe thousands of Rwandan Hutus were systematically massacred by Kabila's forces and Rwandan Tutsi troops when 800,000 were forced home in 1996 and 1997.

Critics have accused the UNHCR of passing the buck in efforts to find out what happened to the missing refugees that were under the agency's protection. The Hutus fled Rwanda in fear of reprisal for 1994's Hutu genocide of minority Tutsis.