Secretary-General Kofi Annan warned Eritrea on Monday that if it continues to impede peacekeeping operations the United Nations may pull its troops out of a buffer zone separating the Eritrean and Ethiopian armies. Eritrea informed the United Nations that it was banning helicopter flights by U.N. peacekeepers in its airspace in the buffer zone starting Oct. 5. It also banned U.N. patrol vehicles from operating at night on its side of the 621-mile Temporary Security Zone. The zone was established after a December 2000 peace agreement that ended a 2 1/2-year border war between the Horn of Africa neighbors.