N'DJAMENA, April 11 (Reuters) - Chad said on Monday it was suspending its mediation in Sudan's Darfur crisis because it could not help a country that was supporting rebels determined to destabilise it.

"We have decided to postpone our mediation because we cannot mediate and accept that the leaders of the country we are helping maintain a rebellion against Chad on their territory," said senior mediator Allam-Mi Ahmad.

Chad, Africa's newest oil producer, accused Sudan last week of seriously threatening its security by recruiting and supplying some 3,000 rebels close to the border between the two countries.

Ahmad said Chad would only agree to resume its mediation once Sudan had tackled the Chadian rebels. "National opinion would not understand that we are helping Sudan make peace while its leaders support people who want to destabilise Chad," he told Reuters.

Tens of thousands have been killed in Darfur and more than 2 million people have fled their homes since rebels took up arms in 2003, accusing Khartoum of giving preference to Arab tribes.

The conflict has weakened relations between the two neighbours and has created political and economic problems for Chad's President Idriss Deby, who has come under pressure from both sides for support.

Deby has hosted peace talks between Darfur's warring parties, to the surprise of some in his Zaghawa tribe, which overlaps the border between the two countries.

Its members are among rebels battling Khartoum in Darfur and Deby, a former army chief who seized power with help from Sudan and Libya in 1990, has faced pressure to back the rebels.

Chad has also accused Sudanese militias of launching cross-border attacks and hundreds of thousands of refugees from Darfur have poured over the border into poor eastern Chad.

An African summit on the Darfur war had been scheduled for April 20 in Egypt, but an official there said on Monday it had been postponed because some leaders could not attend.