ARUSHA, Tanzania — The Canadian general in charge of UN peacekeepers in Rwanda during the 1994 genocide told a UN tribunal Wednesday that the central African country's presidential guard started the massacres that eventually left more than 500,000 people dead.

Under cross-examination by defence lawyer Raphael Constant, retired Lt.-Gen. Romeo Dallaire testified about the first hours of the genocide, which began after the Rwandan president's jet was shot down over Kigali, Rwanda on April 6, 1994.

Constant was acting as the lead counsel for Col. Theoneste Bagosora, the former army chief of staff whom the prosecution alleges was the mastermind of the genocide.

Dallaire said it was clear that troops in Rwanda's Presidential Guard, which Bagasora commanded, were not interested in peace.

"The presidential guards were aggressive and unco-operative," Dallaire said, when Constant asked him about his account of the genocide as written in his book, Shake Hands With The Devil: The Failure of Humanity in Rwanda.

Bagosora, Brig.-Gen. Gratien Kabiligi, Col. Anatole Nsengiyumva and Maj. Aloys Ntabakuze have all pleaded not guilty to charges of genocide and crimes against humanity in the 100-day slaughter of Tutsis and politically moderate Hutus. Dallaire, the 37th person to testify, is considered a key witness in the trial.

Dallaire also testified Wednesday that he did not want Belgian troops as part of the United Nations Assistance Mission to Rwanda because the country was once a Belgian colony.

"But it was the only western country which provided the troops," he said, explaining they were the backbone of the peacekeeping operation.

Presidential Guard troops killed ten Belgian peacekeepers on April 7, 1994, which led Brussels to withdrawal its troops from the country. The UN Security Council ordered almost all of the other peacekeepers to be pulled out two weeks later, on April 21.

Dallaire said Hutu extremists in the government and the governing party thwarted his attempts to stop the genocide. In earlier testimony he said he had alerted his superiors at UN headquarters in New York three months ahead of time that genocide was being planned. However, he was not given permission to pre-empt it.

"My mission was not weak, but was limited," Dallaire replied when Constant asked whether the operation was weak and thus unavoidably led to the violence in Rwanda.

The cross-examination was scheduled to continue Thursday.

The trial of the four Rwandan military officers began in April 2002. The tribunal was set up in November 1994 and has so far convicted 16 people and acquitted one.