KHARTOUM, July 11 (Reuters) - Rebels attacked a town in western Sudan on Sunday, abducting a key tribal leader and two other prominent figures, North Darfur state governor Osman Kibir said.

The rebels snatched the three men in the town of al-La'at in Darfur, stoking further tension in a conflict between Arab militias, the government and the rebels which has created what the United Nations says is the world's worst humanitarian crisis.

The rebels and the government signed a ceasefire agreement in April but both sides say the other has frequently broken it.

The abducted men included al-Sadiq Abbas, the leader of all the Arab tribes in eastern Darfur, a judge and the manager of the town's agricultural bank, Kibir said. He gave no other details of the attack.

Al-La'at is a small town to the east of the North Darfur capital, El Fasher.

Kibir said the rebels, from the Sudan Liberation Movement and the Justice and Equality Movement, had violated the truce in Darfur 50 times since April.

The government is under international pressure to disarm the Arab militias, known as the Janjaweed and is accused by the rebels of conducting a campaign of ethnic cleansing.

German Foreign Minister Joschka Fischer said in Khartoum on Sunday Sudan must honour its promise to the United Nations to disarm the Janjaweed, blamed for violence that has driven around a million people from their homes.

"We appreciate that (U.N. Secretary-General) Kofi Annan and the Sudanese government reached an agreement. But it must be implemented," said Fischer who was on a one-day visit.

Annan secured the promise at talks in Khartoum with President Omar Hassan al-Bashir and other Sudanese leaders last week.

In a statement on Sunday, the Sudan Liberation Movement accused the government of incorporating Janjaweed into the armed forces and police and arming them to fight the rebels and "terrorise innocent civilians".

"We call upon the secretary-general and the U.S. secretary of state to put in place other mechanisms for the disarmament of the Janjaweed and the protection of civilians rather than leaving it to the regime," the rebel statement said.