Two Rwandan Hutu rebel leaders went on trial Wednesday accused of masterminding from their homes in Germany atrocities in the eastern Democratic Republic of Congo (DRC) in 2008 and 2009.

Ignace Murwanashyaka, 47, head of the Democratic Forces for the Liberation of Rwanda (FDLR), and his deputy Straton Musoni, 50, allegedly ordered mass killings and rapes in DR Congo.

The professorial Murwanashyaka and Musoni are facing 26 counts of crimes against humanity and 39 counts of war crimes committed by militias under their command between January 2008 and their arrest in Germany in November 2009.

Minutes after the trial began, the presiding judge called a temporary adjournment to consider a defence motion contesting the composition of the prosecution team.

The United Nations has hailed the trial as a breakthrough after repeated calls by the Security Council to bring FDLR commanders living abroad to justice and bar them from unleashing further violence in the strife-wracked eastern DRC.

A resident of Germany for more than two decades, Murwanashyaka is accused of waging war in DR Congo from his sitting room in the southwestern city of Mannheim, 6,000 kilometres (3,700 miles) away, via telephone calls to his generals.

Musoni, his right-hand man since 2004, moved to Germany in 1986 to pursue his studies.

Prosecutors say Murwananshyaka ordered around 200 killings and "large numbers" of rapes by his militias, had them use civilians as "human shields" and sent child soldiers into battle in eastern DRC.

A UN report counted 240 telephone calls between Murwananshyaka, known as the "The Doctor" for his PhD in economics, and his officers in 2008-2009.

A protege of former DR Congo president Laurent-Desire Kabila, the bespectacled Murwananshyaka also paid occasional visits to his troops in the country's dense forests, according to the UN.

At the time of their arrest, the rebel group said that the two were "in no way involved in the atrocities committed against civilians in eastern DRC," calling their detention "unfair and unjustified."

Murwanashyaka, head of the FDLR since 2001, was among 15 people whose assets were frozen by the Security Council in 2005 on suspicion of involvement in war crimes.

He had been arrested in Germany but was later released in 2006 due to a lack of witnesses for the prosecution. Germany declined a request to extradite him to Rwanda in 2008 but later launched another inquiry.

Investigative journalist Markus Frenzel, who just published a book about international war criminals living in Germany, said pressure from the UN and non-governmental organisations had been decisive in convincing the country to try Murwanashyaka.

"For a long time, (Germany) simply folded its arms and the justice system failed to act," he told AFP.

But he said a damning UN report released in November 2009 had prompted the men's arrest in Germany the same month.

"France, a member of the UN Security Council, had a preliminary version of this explosive report and passed it on to the Germans," Frenzel said.

"It said it all -- that one of the worst wars on the planet was being directed from Germany, that the telephone calls prove it and that FDLR money was passing through bank accounts in Germany."

The prosecution case cites some 80 witnesses, many interviewed on the ground in eastern DR Congo about atrocities allegedly committed by Murwananshyaka's troops.

The FDLR was created by the perpetrators of the 1994 Rwandan genocide, who fled to neighbouring eastern DR Congo after President Paul Kagame took power.

Numbering up to 5,000, the militia is seen as a major source of instability in Africa's resource-rich Great Lakes region.

The trial before a special court in the southwestern city of Stuttgart is set to run until at least July.