DAR ES SALAAM (Reuters) -- Four former ministers go on trial on Monday charged with playing key roles in Rwanda's 1994 genocide, including handing out weapons, traveling abroad to buy guns and inciting the slaughter of 800,000 people.

The UN tribunal, based in the northern Tanzanian town of Arusha, is keen to show progress in trying former top officials to counter Rwandan government criticism that it has been slow to bring the masterminds of the massacres to justice.

"The crimes they are alleged to have committed resulted in massacres against Tutsis and moderate Hutus," Roland Amoussouga, spokesman for the U.N. International Criminal Tribunal for Rwanda (ICTR) said on Sunday.

The ministers belonged to an interim government that came to power in April 1994 after a plane carrying President Juvenal Habyarimana was shot down just before the killing started.

The interim government was ousted by Tutsi-led rebels three months later, but by then government-sponsored Hutu militants had killed an estimated 800,000 ethnic Tutsis and moderate Hutus.

The ministers served under Prime Minister Jean Kambanda, who became the first head of government to be convicted of genocide when he was jailed for life by the tribunal in September, 1998.

Among the defendants due to go on trial on Tuesday is former Health Minister Casimir Bizimungu, a 52-year-old former doctor who studied medicine in the United States.

Bizimungu, who was arrested in Kenya in February 1999, denies charges of genocide, incitement to genocide, crimes against humanity and war crimes.

He is accused of traveling overseas to buy weapons for the militias with government funds and of doing nothing to stop massacres of patients and staff at hospitals under his control.

Appearing with him are former Trade and Industries Minister Justin Mugenzi, former Minister of Foreign Affairs and Cooperation Jerome Bicamumpaka and former Minister for Civil Service Prosper Mugiraneza. They also deny the charges.

Prosecutors say that as early as April 9, two days after the massacres began, Mugenzi, a 54-year-old former businessman, openly expressed satisfaction that many Tutsis had been killed.

Bicamumpaka, Mugenzi and Mugiraneza were arrested in Cameroon in April 1999 and later transferred to the ICTR.

The tribunal, set up in November 1994, has sentenced 12 people so far, four of whom are appealing against their convictions. More than 40 suspects are in custody.