(Adds details of evacuations, background)

KHARTOUM, Dec 30 (Reuters) - Sudan has arrested six members of the Darfur rebel Justice and Equality Movement (JEM) serving as observers with the African Union (AU) from an AU peacekeeping base, rebels said on Sunday.

The move follows an attack by JEM on Suleia town in West Darfur on Saturday, and as some humanitarian workers are being evacuated from Sudan's border area with Chad amid increased tensions.

"They were taken by the government in el-Fasher from inside the headquarters of the African Union base in front of their eyes," JEM leader Khalil Ibrahim told Reuters from Darfur.

The six were monitors assigned to maintain contacts with the African Union, as part of a stalled ceasefire commission which also included Sudan's army as well as other rebel groups to investigate truce violations.

"They (the AU) failed to protect them," he said.

A senior AU official confirmed the detentions and said discussions were under way with the Sudanese government on the situation.

The United Nations said up to 16 humanitarian workers had been evacuated from two towns in West Darfur following operations by the Chadian army in the sensitive border area.

"There were three staff members from WFP (World Food Programme) and UNHCR (the U.N. refugee agency) and between 11-13 NGO staff members," U.N. spokeswoman Radhia Achouri said.

She added those working in the world's largest aid operation in Darfur were being advised not to travel to el-Geneina as a precautionary measure.

"The situation is a little bit volatile," she said. Sudan accused Chad of bombing two areas in Darfur on Dec. 28, a charge N'Djamena denied.

More than 12,000 aid workers -- mostly Sudanese -- help more than 4 million Darfuris in need after five years of revolt in Sudan's west. International experts estimate some 200,000 have died, while Khartoum puts the death toll at 9,000. (Reporting by Opheera McDoom; editing by Sami Aboudi)

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