The Canadian general in charge of United Nations peacekeepers in Rwanda during the 1994 genocide testified Monday that one of the Rwandan officers accused of masterminding the mass slaughter threatened to kill him.

Retired general Roméo Dallaire said that former Rwandan army colonel Theoneste Bagosora threatened him "with a pistol and was told that next time he will kill me."

Dallaire, who warned the UN about the potential for mass killings months before they happened, is testifying Monday and Tuesday at the international tribunal in Arusha, Tanzania. The tribunal began in April 2002.

Romeo Dallaire in Arusha, Tanzania (AP photo)

Bagosora and three other former Rwandan army officers have pleaded not guilty to genocide, conspiracy, war crimes and crimes against humanity.

The massacres began after the Rwandan president's plane was shot down over the capital on April 6, 1994.

Dallaire said Bagosora took control of the country and also chaired military meetings April 7 and 8.

He said that led him to believe that Bagosora was behind the killings.

"This is extremely crucial for the prosecution to have a witness of the calibre of General Dallaire taking the stand telling the whole world what it was like dealing with each of the accused persons," said Roland Amoussouga, a spokesman for the International Criminal Tribunal for Rwanda.

The former colonel is at the centre of Dallaire's memoir, Shake Hands With the Devil.

In the book, Dallaire accuses Bagosora of orchestrating the death squads.

The prosecution alleges that Bagosora ran the slaughter from his office in the Ministry of Defence.

The 100-day genocide killed more than 800,000 people, mostly minority Tutsis and politically moderate Hutus.

Several months before the slaughter, Dallaire warned the UN about death squads and plans for mass killings.

Dallaire took early retirement in April 2000 because of post-traumatic stress disorder and has complained of nightmares and flashbacks.

In 1998, Dallaire testified at the trial of Jean-Paul Akayesu, former mayor of Taba in central Rwanda. Akayesu was convicted of genocide and sentenced to life imprisonment.