If Iraqi President Saddam Hussein is toppled from power, U.S. military commanders will take early responsibility for delivering security, destroying weapons of mass destruction and providing emergency food to millions of vulnerable Iraqis, senior Bush administration officials said yesterday.

In the administration's first public explanation of its plans, the officials sketched for Congress an ambition to lead the rebuilding of Iraq's battered economy and the reshaping of society, a project they said would cost untold billions and take years to complete.

Simply discovering and destroying Iraq's stockpiles of biological, chemical and nuclear materials -- the primary goal of a prospective invasion, according to President Bush -- would be a "huge undertaking," Undersecretary of Defense Douglas J. Feith told the Senate Foreign Relations Committee.

"We cannot now even venture a sensible guess as to the amount of time," he said. To the frustration of several senators, Feith and Undersecretary of State Marc Grossman declined repeatedly to discuss cost estimates for a prospective war or its aftermath, asserting that too much remains unknown.

The hearing focused attention on post-conflict problems that have received little public attention, yet would likely be crucial to the long-term success of any U.S.-led effort to replace Hussein.

Worries about a potential humanitarian crisis have grown as the administration's deadline draws nearer. The United States, the United Nations and international organizations are hurrying to build networks to care for refugees and deliver food to millions of Iraqis whose lives could be turned upside down.

Tents, medicine, food and water-purification systems are among supplies being stockpiled in countries neighboring Iraq, while teams inside and outside government are laboring over the complex logistics of relief and reconstruction. A steady stream of U.S. officials is flowing through the region to study transport routes and coordinating details with Iraq's neighbors.

However, planners say funding is not arriving fast enough. The United Nations, which struggled to raise $37 million it requested from donors in December, was seeking more than $125 million at last count. International agencies say they are ill-prepared to handle large-scale trouble in a country where few humanitarian groups operate.

"Everyone is holding their breath," Ruud Lubbers, the U.N. High Commissioner for Refugees, said. "We need some signs of international burden-sharing."

The Bush administration, which has contributed about $25 million, will be seeking much more from Congress for humanitarian aid and other war costs, according to U.S. officials and congressional staff members. A Capitol Hill aide said he was told that Congress would get "sticker shock."

At yesterday's Senate hearing, Ohio Republican George V. Voinovich said the administration must "forthrightly" deliver budget numbers. Sen. Joseph Biden (D-Del.) said Americans "have no notion about what they're about to undertake." He said Bush must remain committed to the rebuilding effort if he launches a war, even if it means limiting his proposed tax cut or curtailing his domestic agenda.

Senators from both parties expressed strong doubts about the White House's readiness to solve Iraq's myriad problems. Several said the administration has been too slow to reveal its plans and questioned whether hard-line tactics at the U.N. Security Council were alienating prospective partners in the reconstruction effort.

"Why aren't we hearing some more about a worst case? Do you have some kind of plan if this turns into a debacle?" asked Sen. Lincoln D. Chafee (R-R.I.). Feith said the administration does, but did not elaborate.

Grossman and Feith said the Americans aim to enter Iraq as liberators and depart after helping to create a stable, democratic government on a path toward economic security. The Pentagon would play the lead role in relief and reconstruction efforts, but would hope to relinquish some authority quickly to Iraqi ministries, international organizations and relief groups. Grossman estimated that Iraqis could regain control of the country in two years.

Retired Army Lt. Gen. Jay M. Garner, who commanded security operations in northern Iraq after the 1991 Persian Gulf War, has been named as coordinator for reconstruction and humanitarian assistance. He would report to Gen. Tommy R. Franks, commander of U.S. forces in the region, who would be the top-ranking U.S. official in postwar Iraq.

U.N. staff members inside Iraq believe that in the case of war, 2 million people may leave their homes, in addition to about 1.1 million Iraqis already displaced, according to a confidential Jan. 16 draft of the U.N.'s Integrated Plan. Lubbers's refugee office estimates 600,000 Iraqis could push across the country's borders if war breaks out -- a number that could grow if the conflict dragged on or Hussein used chemical or biological weapons.

The Bush administration plans to initiate relief efforts in some parts of the country within days of any invasion. In one often-discussed scenario, Iraqi army units in the north and far south of the country would collapse or retreat, allowing U.S.-led forces to move on Baghdad, where Hussein's elite units are concentrated. This would open space for early relief work at the local level.

Yet a number of U.S. organizations have complained that Pentagon planners have not provided enough information or established realistic expectations. More broadly, Refugees International Vice President Joel R. Charney recently called international planning for a humanitarian calamity "woefully inadequate."

Aid groups including the umbrella organization InterAction have spent months lobbying the Treasury Department, for example, to issue licenses to groups anxious to develop contingency plans in northern Iraq and neighboring Iran. Word came only late last week that the licenses would be issued.

"Is the U.S. military really prepared to feed more than 14 million people that the U.N. says are dependent on international aid?" asked Sandra Mitchell, of the International Rescue Committee. "And is the U.S. military prepared to protect the lives of more than 23 million Iraqis if they invade?"

There is wide agreement that retaining Iraq's food distribution system would be essential to keeping Iraqis fed and helping to prevent a breakdown in civil order. A network of more than 40,000 distributors monitored by the United Nations delivers food and basic supplies to every Iraqi. The United Nations estimates that about 60 percent of Iraqis are dependent on the food they receive.

Sources confirm that the World Food Program has been involved in contingency discussions about managing the project if war comes to Iraq, although no agreement has been signed.

If the WFP took over the food distribution network, the agency would need at least $40 million to hire staff, procure fuel and pay the salaries of Iraqi workers for about two months, according to one U.S. official. The WFP would also need more than $400 million buy food before the agency could gain access to about $2.5 billion in food supplies now in the Iraqi pipeline. The United States would be expected to advance 25 to 30 percent of that money, the official said.

In a sequence under evaluation by U.S. planners, Iraqis would rely on previously distributed rations during roughly the first month of a war. The Iraqi government has been issuing double or triple rations for several months in anticipation of a conflict, but the United Nations has predicted the Baghdad government would interrupt distributions during a war.

During the second month, Iraqis would be able to draw on food now being sent to the region by the U.S. government, as well as supplies provided under existing emergency agreements with such countries as Jordan and Syria, the official said. By the third month, if all went well, the WFP would have the currently functioning distribution system up and running.

The Bush administration is counting on the Iraqi bureaucracy to play a significant role in reconstruction -- minus senior officials deemed corrupt or unreliable. Feith said U.S. analysts are vetting current Iraqi officials to identify likely partners. He said the administration will not quit on the Iraqis.

"As long as it takes," Grossman said, "that's what it will take."