With its workers facing a barrage of sniper fire and artillery attacks from forces loyal to Ivory Coast strongman Laurent Gbagbo, the United Nations on Sunday ordered the “temporary relocation” of 200 U.N. civilian staff members from their headquarters in Abidjan but vowed to continue its mandate to protect civilians.

The move marked the most significant retreat by the 9,000-strong U.N. peacekeeping mission in Ivory Coast, known as UNOCI, since the country emerged from a civil war in 2002. The U.N. mission is responsible for helping to guarantee the country’s political transition and to protect civilians under imminent threat of attack. But the mandate was created in a time of peace, and the mission has strained to fulfill its goal as the crisis has deepened into an all-out civil war.

In a sign of growing international alarm at the violence, France seized control of the country’s principal airport and reinforced its military presence in the nation with 400 additional troops, bringing their total number to nearly 1,500. The operation is designed to ensure the protection and possible evacuation of foreign nationals, including more than 1,600 sheltered in a French military camp. But Gbagbo’s supporters accused the former colonial power — which has kept a small force in Ivory Coast to support the U.N. peacekeepers — of seeking to reoccupy the country.

The escalating violence comes more than four months after Gbagbo rejected the outcome of the Nov. 28 presidential runoff, which opposition leader Alassane Ouattara won. Since then, forces loyal to Gbagbo have launched a campaign of violence against Ouattara’s supporters and the U.N. in an effort to consolidate his hold on power.

Several months of diplomatic efforts, led by African governments and backed by Western financial sanctions, to nudge Gbagbo from power have failed to persuade him to leave. Last month, a coalition of former Ivorian rebels and disaffected police and soldiers launched a military offensive to oust Gbagbo and allow Ouattara to assume power.

The military advance, which began in northern Ivory Coast, met limited opposition as it swept across the country and penetrated the commercial capital of Abidjan last week, where fighting has deadlocked. In a sign of Gbagbo’s growing isolation, his government’s minister of cooperation and foreign affairs, Bamba Mamadou, sought refuge at a U.N.-controlled facility last week, according to a U.N. official.

Alarmed by reports of atrocities, U.S. Secretary of State Hillary Rodham Clinton urged the U.N. to continue to “aggressively enforce its mandate to protect civilians,” and voiced concern over the “dangerous and deteriorating” security situation.” Clinton cited reports of “gross human rights abuses” by both sides in the conflict and “potential massacres” in the west, where U.N. officials and aid workers say hundreds of people were killed in a massacre last week.

“The United States calls on former President Laurent Gbagbo to step down immediately,” Clinton said in a statement. “His continuing refusal to cede power to the rightful winner of the November 2010 elections, Alassane Ouattara, has led to open violence in the streets, chaos in Abidjan and throughout the country, and serious human rights violations. Gbagbo is pushing Cote d’Ivoire into lawlessness.”

Hamadoun Toure, the U.N. spokesman in Abidjan, said the U.N. had decided to relocate 200 nonessential U.N. personnel to the northern city of Bouake after the headquarters came under sniper fire from Gbagbo’s special forces and endured shelling from an armored personnel vehicle.

Toure said 11 U.N. peacekeepers were injured in attacks by pro-Gbagbo forces last week. He said that the U.N. has been sending more “robust” military patrols into the streets of Abidjan but that “it’s very difficult because they are shooting at us.” However, he added, a full withdrawal by the U.N. is “out of the question.”

U.N. officials and human rights workers maintain that Ouattara’s forces have also engaged in human rights abuses. On Saturday, the U.N. mission confirmed that at least 330 people had been killed in the western town of Duekoue, including 230 killed by Ouattara loyalists and 100 killed by Gbagbo’s fighters.

In a telephone conversation with Ouattara late Saturday, U.N. Secretary General Ban Ki-moon “expressed particular concern and alarm about reports that pro-Ouattara forces may have killed civilians in the town of Duekoue in the west of the country,” according to a U.N. account of the discussion. “President Ouattara, while denying his forces were involved, said he had launched an investigation and would welcome an international inquiry into the matter.”

Clinton joined Ban in appealing to Ouattara to hold his forces accountable for rights abuses. They must “respect the rules of war and stop attacks on civilians,” Clinton said. “As President Ouattara takes the reins of government, he must prevent his troops from carrying out reprisals and revenge attacks against their former foes. The people of Cote d’Ivoire await and deserve the peace, security, and prosperity he has promised, and that they have for so long been denied.”