By Michelle Nichols

UNITED NATIONS (Reuters) - Four United Nations civilian staff have returned to a U.N. peacekeeping mission in disputed Western Sahara after months of talks between the world body and Morocco to defuse tensions over Secretary-General Ban Ki-moon's use of the word "occupation" to describe Rabat's annexation of the territory in 1975.

Morocco expelled dozens of U.N. civilian staff working for the mission known as MINURSO after Ban made the remark in March.

Last month, Morocco proposed allowing 25 civilian staff to return.

U.N. spokesman Nick Birnback said on Thursday that the first group of staff members landed on Wednesday in Laayoune, Western Sahara's largest city. "Additional staff members will be returning in the coming days," Birnback said.

A U.N. source, speaking on condition of anonymity, said that so far four staff had returned.

The controversy over Ban's comment during a visit to refugee camps for Sahrawi people is Morocco's worst dispute with the United Nations since 1991, when the U.N. brokered a ceasefire to end a war over Western Sahara and established MINURSO.

Morocco's government spokesman Mustapha Khalfi declined to confirm the return of U.N. civilian staff.

"The kingdom had assured that it wants the MINURSO to do its job and that is why we started talks with the U.N.," he said. "We will announce the results (of the talks) at the right moment."

In April the U.N. Security Council extended MINURSO's mandate for another year and demanded urgent restoration of all of its work. Council diplomats and U.N. officials said, however, that discussions on making MINURSO fully functional again have been slow and difficult. Before the staff reductions, MINURSO had nearly 500 military and civilian personnel.

The Sahrawi people's Polisario Front movement, which demands self-determination for Western Sahara, wants a referendum on independence. Morocco, which took over Western Sahara from colonial power Spain, says it will only grant autonomy.

(Additioal reporting by Aziz El Yaakoubi in Rabat; editing by Grant McCool)