UN spokesman Stephane Dujarric said the UN Mission in Sudan (UNMIS) will reduce its presence and restrict access in parts of west Darfur.
Mr Dujarric said it was: "due to the increasing instability in the affected areas, including a buildup of forces on either side of the Sudan-Chad border with increased potential for armed conflict."
But he said the move by the 5,783-strong UNMIS did not signal an overall evacuation from the area.
"Essential life-saving humanitarian services delivered by the UN will continue and the mission will monitor the situation and carry out a fresh security assessment of the affected areas in the next two to three weeks," said Mr Dujarric.
UNMIS chief spokesman George Somerwill said from Khartoum that about 100 personnel were in the affected areas before the staff cut took effect over the last two to three months. But he would not say how many people had been withdrawn.
"It's not a big reduction," he said, adding that those left behind were humanitarian workers tasked only with responding "to the life-threatening needs of people in the area."
"There is a heavy buildup of troops on both sides of the border. It has been quite bad in the last three weeks," Mr Somerwill noted." "I think there is some cause for concern."
Mr Dujarric said the escalating tension along the Sudanese-Chadian border would be discussed when Chadian Foreign Minister Ahmad Allam-mi visits UN headquarters next week to attend a UN Security Council meeting on Sudan.
Chad accuses Sudan
Chadian President Idriss Deby recently accused Sudan of "exporting" the Darfur crisis to his country, as Central African leaders met at a crisis summit in Ndjamena over escalating tensions between the neighboring states.
"The Khartoum regime is secretively going ahead with the recruitment of mercenaries and other elements to put into action its Machiavellian plan – the destabilisation of Chad," Mr Deby said in opening remarks to the Central African Economic and Monetary Community (CEMAC) mini-summit.
Mr Deby has repeatedly accused the Khartoum regime of supporting Chadian rebels in eastern Chad, on the border with Darfur, which has been in the throes of a civil war for three years.
Several new rebel groups have sprung up recently in eastern Chad, which houses more than 200,000 refugees who fled Darfur.
The CEMAC leaders of central African states gave qualified support to Chad in its dispute with Sudan and condemned any attempt to destabilise the country.
War broke out in Darfur in early 2003 when rebel groups began fighting what they say is the political and economic marginalisation of the region's black African tribes by the Arab-led regime in Khartoum.