ABIDJAN, Ivory Coast - The United Nations said Sunday that it had received hundreds of reports of people being abducted from their homes at night by armed assailants in military uniform and that there is growing evidence of "massive violations of human rights" since Ivory Coast's disputed presidential election.

The statement from the Office of the U.N. High Commissioner for Human Rights came a day after the world body said it will not remove its thousands of peacekeeping troops from the country, as demanded by Laurent Gbagbo, the incumbent who the international community contends lost the election.

Navi Pillay, the top U.N. human-rights official, said Sunday that more than 50 people had died over the last three days in election-related violence, up from previous estimates of as many as 30 people.

"The deteriorating security conditions in the country and the interference with freedom of movement of U.N. personnel have made it difficult to investigate the large number of human rights violations reported," Pillay said in a statement released from her office in Geneva.

It also said that the armed assailants responsible for the abductions had been "accompanied by elements of the Defense and Security Forces or militia groups."

International pressure is mounting for Gbagbo to concede defeat to opposition leader Alassane Ouattara, whose victory has been recognized by the United Nations, the United States, former colonial power France, and the African Union.

The State Department on Sunday ordered most of its personnel to leave Ivory Coast because of the deteriorating security situation and anti-Western sentiment.

U.S. officials also warned American citizens to avoid travel to the West African nation until further notice.

The European Union had given Gbagbo until Sunday to concede defeat or face sanctions that would include an assets freeze and a visa ban on him and his wife. The U.N. Security Council is also expected to meet Monday to discuss Ivory Coast's political crisis.

Still, experts say there are few strong options for forcing Gbagbo from office, and it is unlikely the African Union or others will back military intervention.

"The trouble is, both sides are clearly preparing now for conflict, and a cornered Gbagbo shows little sense of the national tragedy unfolding through his brinkmanship," said Alex Vines, head of the Africa program at Chatham House, an independent research center in London.

In a statement read on state television Saturday, Gbagbo's spokeswoman said that 9,000 U.N. peacekeepers and 900 French troops supporting them must leave the volatile West African country immediately. Gbagbo accused the U.N. mission of arming rebels who support Ouattara.

The U.N. had been invited by the government to supervise the vote and certify the outcome following a peace accord after Ivory Coast's 2002-2003 civil war.

About 800 of the U.N. peacekeepers are protecting a hotel compound from which Ouattara is trying to govern the country. They are in turn encircled by Gbagbo's troops.

U.N. Secretary-General Ban Ki-moon said Saturday that the U.N. mission known as UNOCI would stay in Ivory Coast despite Gbagbo's demand.