U.N. Secretary General Kofi Annan on Monday proposed expanding the peacekeeping force in Congo from 10,800 to nearly 24,000 troops, making it the largest and costliest U.N. mission in the world.

The move comes as the United Nations-backed peace process in Congo is facing increased challenges from armed groups seeking to disrupt the country's fragile political transition. It is designed to present a greater international show of force as the country moves closer to elections in July 2005.

The enlarged mission calls for the deployment of thousands of fresh U.N. troops in Congo's most violent areas, including Ituri, the North and South Kivu, and Katanga. Annan said in a 33-page report to the Security Council that he will need an additional 37 attack, support and reconnaissance helicopters, along with two transport planes, to support a new rapid-reaction force that can travel to trouble spots.

The Bush administration reacted cautiously to the request, which could more than double the $700 million annual cost of funding peacekeeping in Congo. The United States is assessed 27 percent of the total.

"We have many questions for the United Nations, and we would like to understand the basis for their substantial request," said Richard Grenell, a spokesman for the U.S. mission to the United Nations.

"We obviously agree that adjustments are necessary," he added. "We agree that a more mobile force that is capable of rapid response is needed, but it's important to remember that there is no peacekeeping operation on its own that can possibly guarantee security everywhere. We've said all along that we think it's important for the Congolese political actors to do what they can to create a stable environment."

U.N. officials said they anticipate little trouble recruiting peacekeepers from poor countries, including Pakistan. But Annan said he will need commitments from wealthy Western nations to provide advanced aircraft, communications and other logistical equipment.

"Countries from the developing world have been providing most of MONUC's military assets since the inception of the mission," Annan wrote. "But the Security Council should not rely only on troop contributors from developing countries. Other troop contributing countries must also play an active role in assisting the Congolese peace process."

The former Belgian colony of Congo, once called Zaire, has been the scene of some of Africa's worst violence since its former ruler, Mobutu Sese Seko, was overthrown by rebel forces led by Laurent Kabila. In 1998, it set the stage for the continent's deadliest regional war as Kabila's erstwhile sponsors, Rwanda and Uganda, sought to drive him from power.

Angola, Namibia and Zimbawe entered the war on behalf of the Congolese leader. Under a 1999 peace accord, the five countries agreed to withdraw their troops from the country. U.N. peacekeepers sent to Congo to help implement the accord have faced a series of challenges from government troops and scores of armed groups, including some who have received the support of Congo, Rwanda and Uganda.

The report expressed "deep concern" over rampant human rights abuses in Congo since late March, citing cases of mass rape, looting, abductions and executions by scores of armed militias. It also noted that government forces have detained civilians, holding them in illegal underground cells, and "frequently" turned to crime to make a living.

In early June, one Rwanda-backed rebel group led by two renegade Congolese officers temporarily seized control of the strategically important town of Bukavu. A force of 400 U.N. peacekeepers based in Bukavu was unable to repel the attack.

Annan sought to dampen expectations that the reinforced U.N. peacekeeping mission would be robust enough to impose peace in Congo, a country of 58 million people that spans a territory the size of Western Europe. But he suggested that the beefed-up force could "deter spoilers from derailing the transition."