(Adds details, U.N. humanitarian affairs official)

UNITED NATIONS, March 5 (Reuters) - U.N. aid operations in Darfur will be severely damaged if the Sudanese government follows through on its decision to expel aid organizations from Darfur, U.N. Secretary-General Ban Ki-moon said on Thursday.

"The decision by the government of Sudan to expel 13 non-governmental organizations (NGOs) involved in aid operations in Darfur will, if implemented, cause irrevocable damage to humanitarian operations there," Ban said in a statement read out by his spokeswoman Michele Montas.

"These agencies are key to maintaining a lifeline to 4.7 million Sudanese people who receive aid in Darfur," the statement said, adding that Ban "appeals to the government of Sudan to urgently reconsider the above decision."

Khartoum's decision to expel aid agencies from Sudan's western Darfur region followed the indictment on Wednesday of President Omar Hassan al-Bashir for war crimes in Darfur, making him the first sitting head of state to be charged by the International Criminal Court in The Hague.

Deputy U.N. humanitarian chief Catherine Bragg told reporters that the United Nations had received reports that some NGO workers were being harassed and detained by Sudanese security forces.

"We have reports of a number of international staff of NGOs who were detained for up to four hours," she said, adding that some security forces were reportedly "very intimidating, very aggressive."

Ban said in his statement that "the confiscation of equipment, money and other materials (from NGOs) is unacceptable and must end immediately."

Bragg said that there are 76 NGOs working in Darfur alongside the U.N. staff. (Reporting by Louis Charbonneau; editing by Mohammad Zargham)

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