THE blue and white flag of the United Nations may soon be flying over the besieged Adriatic port of Dubrovnik if a project to safeguard the historic walled city attains its goal.With the famed Renaissance "old town" figuring on the international organization's World Heritage list of irreplaceable cultural and natural sites, UNESCO Director-General Federico Mayor dispatched two observers yesterday to assess the damage Dubrovnik has suffered in Yugoslavia's four-month civil war. An unstated goal of the UNESCO mission is to prevent extensive damage to Dubrovnik, a city so beautiful even Napoleon is said to have vowed to spare it in battles for the Dalmatian coast. The mission was undertaken only with pledges of full cooperation from Croatian forces and Serbian leaders of the federal Army. "We are particularly conscious of our limits, but we shouldn't overlook the psychological impact this kind of intervention can have," says Luis Marques, deputy director general for external relations of the Paris-based United Nations Educational, Scientific, and Cultural Organization. UNESCO officials are hoping a UN presence within Dubrovnik's walls will reinforce commitments from Serbian military leaders to spare the city. The two observers plan to raise the UN flag on the walled city's historic Minceta Tower. Despite widespread press reports claiming the contrary, Dubrovnik's old town has suffered no "irreversible damage," according to UNESCO officials. The city's mayor and Mr. Mayor have kept in daily contact via facsimile machine since intense fighting for Dubrovnik began in October. "A certain number of buildings have been affected, as have the ramparts," says Daniel Janicot, Mayor's cabinet director, "but as of now, damage is very limited." If the two observers are able to enter Dubrovnik, they will prepare a complete inventory of damage and recommend measures needed for restoration. Mayor has worked to express UNESCO's concern for the people caught in the civil war, and officials reemphasized that position this week. "We do not overlook the human situation of this war, with perhaps 10,000 dead" and hundreds of thousands of injured and homeless, says Mr. Janicot, "that must come first."