Top U.N. ambassadors were forced to hitch a bus ride from Democratic Republic of Congo to Rwanda on Sunday after a security guard accidentally shot a hole in their plane.

The Security Council ambassadors had been visiting violent eastern Congo during a trip to promote peacekeeping operations and other efforts to end some of the most intractable conflicts in Africa.

"They were boarding the aircraft to come back to Kinshasa. U.N. security have to surrender their weapons on the plane. He (a guard) was doing a safety check and there was an accidental discharge," said Kemal Saiki, spokesman for the U.N. peacekeeping mission in Congo, the world body's biggest.

"The crew thought that something may have been damaged so they decided not to fly," Saiki said, speaking in Goma, Congo.

The head of the U.N. mission in Congo, Alan Doss, told reporters that an investigation would be undertaken.

Other eyewitnesses at the scene said the bullet, which made a loud bang inside the aircraft, went through the floor of the plane and touched a cable.

The envoys and journalists accompanying them travelled by bus across the border to Rwanda, where their plane was to pick them up, Saiki said.

After a four-hour bus ride through the mountainous terrain of Rwanda, the delegation was stuck for hours at the Kigali airport after U.S. aviation fuel firm Caltex demanded $20,000 up front for fuel from the group.

At first, ambassadors and reporters began digging in their pockets to see if they could come up with the money in cash to pay for the fuel and leave Rwanda. But the United Nations ended up transferring the funds by wire to Caltex.

Several envoys said they were furious about the delay and would consider formally complaining to the company, which one ambassador said was planning to shut down its Rwandan operations in the near future.

Company officials were not available for comment.

The envoys are due to fly on to Ivory Coast where the envoys were intending to express support for the country in a presidential election on November 30.

The plane the envoys flew in to Kinshasa was too big to land safely in Goma in eastern Congo since lava from a volcanic eruption a few years ago cut the runway short.

The Security Council envoys' Africa visit, which has become an annual fixture in recent years, is often plagued by organisational hitches and last minute itinerary changes as the diplomats try to cram in visits to the continent's worst trouble spots.

Chadian President Idriss Deby failed to show for a scheduled meeting on Friday to discuss his feud with neighbouring Sudan which has endangered aid operations helping hundreds of thousands of refugees in and around Sudan's Darfur region.