CANBERRA -- The head of a United Nations court trying suspects of Rwanda's 1994 genocide warned on Wednesday the trials could grind to halt because witnesses are being blocked from appearing.

A lack of witnesses has put nine current trials on hold, including that of former cabinet minister Eliezer Niyitegeka who is accused of 10 counts relating to genocide, murder and rape in the 100-day killing spree when 800,000 Tutsis and moderate Hutus were massacred told Reuters.

The Court's Chief Prosecutor, Carla del Ponte, has told the UN Security Council that Rwanda has withdrawn all cooperation and prevented witnesses attending the Tanzania-based court, due to plans by prosecutors to investigate present leaders.

Judge Navanthem Pillay, the President of the International Criminal Tribunal for Rwanda, said she wrote to the UN Security Council, warning that without witnesses the tribunal could grind to a halt when it returns from recess on August 13.

"At this stage it is uncertain whether the trials can continue," Pillay told Reuters in an interview in the Australian capital where she is the guest of the center for international and public law.

Pillay said she hoped the 15-Nation Security Council would talk to the Rwandan government to ensure the trials proceeded and the United Nations could continue to offer witnesses anonymity and transport to court hearings.

"We've had two United Nations planes come back empty recently and that costs money," she said.

Rwanda's Chief Prosecutor, Gerard Bahima, told the UN last week that his government had not stopped genocide survivors giving evidence but it opposed prosecution of army soldiers and officers, saying such cases should be tried by national courts.

Gahima said the tribunal was too slow and failed to protect witnesses.

--- Slow Process --- The tribunal was set up in 1995 to try the main architects of a three-month killing spree when ethnic Hutu extremists murdered Tutsis and politically moderate Hutus in Rwanda, a former Belgian Colony in Central Africa.

Despite a budget of $100 million a year and 800 staff, only eight people have been convicted so far, including former prime minister Jean Kambanda who received life imprisonment.

Currently 24 more people are before the courts and 29 others are in custody awaiting trial.

Pillay, a South African judge of Indian descent, said the prosecutor had just drawn up a list of another 136 wanted individuals, with plans for the tribunal to handle 24 of these and seek other countries willing to prosecute the others.

With the tribunal's workload still high, Pillay has tried to speed up the court's work since she was elected president in 1999. She is waiting to hear from the Security Council on a plea she made a year ago for 18 extra judges.

"With those extra judges we could complete our work by 2007/08," said Pillay, who intends to leave the tribunal when her four-year term expires next May. "This is important...because of the long awaiting trial period being endured by people in detention and because we worry about the liability of testimony many years after the event."