KHARTOUM, May 21 (Reuters) - Sudan will ease aid workers' travel to its western Darfur region, where U.N. officials say one of the world's worst humanitarian crises is unfolding, by temporarily lifting the need for travel permits.

U.S. officials have complained that access to nearly one million people displaced by fighting in Darfur has been prevented by bureaucratic delays and time limits on Sudanese visas and travel permits to the remote region bordering Chad.

Rebels took up arms in Darfur in February last year, accusing the government of neglecting the poor area and arming Arab militias to loot and burn the villages of ethnic Africans. Khartoum denies the charge, saying the militias are outlaws.

But in a joint statement issued late on Thursday, the Ministry of Foreign Affairs and Ministry for Humanitarian Affairs said aid workers going to Darfur would not need travel permits for the next three months, as of Monday.

"The government has decided to grant those working in the humanitarian field in Darfur, the representatives of U.N. agencies, the Red Cross, the donor (countries) and voluntary non-governmental organisations, instant entry visas from its diplomatic missions abroad within 48 hours of the date of the visa request," the statement said.

It added that the visas would be valid for three months.

The United Nations estimates more than 120,000 refugees are also encamped in neighbouring Chad. Aid workers say more arrive daily as rebels accuse the government of violating a ceasefire signed in early April to allow in humanitarian aid.

Darfur will top the agenda of 15 African leaders who are due to meet in Addis Ababa on Tuesday, an African Union (AU) official said in the Ethiopian capital, Addis Ababa, on Friday.

Said Djinnit, an AU commissioner for the 15-nation peace and security council, told a news conference the council was unable to quell the crisis in Darfur and other parts of the continent "due to lack of capacity and resources".

The council was directed by the 53-nation AU at its inception in February to set up a multinational force and to intervene across the troubled continent to end civil unrest.

Peace talks are continuing in Kenya to end 21 years of a separate civil war in southern Sudan. The talks do not cover the Darfur conflict.

(Additional reporting by Tsegaye Tadesse in Addis Ababa)