GENEVA (Reuters) - Major aid agencies Friday warned of an "unprecedented disaster" in Iraq as they swung into action to avert food shortages and disease outbreaks among an already impoverished and underfed population.

With relief operations being hamstrung by donor reluctance to contribute funds ahead of the conflict, the United Nations said it will launch an appeal for more than $1 billion early next week.

U.S. forces thrust deep into southern Iraq a day after launching air strikes and the United States said it still hoped to topple President Saddam Hussein without an all-out war. Scores of Iraqis have surrendered and some have been killed.

The International Committee of the Red Cross (ICRC) said its officials were making the rounds of hospitals in Baghdad Friday after a second night of bombing.

Doctors Without Borders, which along with the ICRC deploys virtually the only expatriate aid workers still in Baghdad, said it had no reports of major casualties following the one death and 14 injured reported Thursday. It said Baghdad hospitals and emergency medical facilities still appeared to function.

But agencies sounded the alarm about a civilian population already weakened by the impact of nearly 13 years of economic sanctions. Almost two-thirds of the Iraqi population relies on rations for all their food. Malnutrition already strikes 1.3 million Iraqi children under the age of 5, or one in four, according to the UN's Children's Fund UNICEF.

"Iraq is on the brink of an unprecedented humanitarian catastrophe. We are facing our largest and most complex operation ever undertaken," spokeswoman Wivina Belmonte said.

MILK AND COOKIES

UNICEF has brought milk and high-protein cookies into Iraq for 240,000 malnourished children and 130,000 pregnant and nursing women. It has repaired generators for 100 water and sewage facilities but says 5 million people are "at risk because of a lack of access to water and sanitation services."

The UN's World Food Program (WFP), which estimates that Iraqis have about six weeks worth of food reserves left, has already brought 32,000 tons of food to surrounding countries. It is seeking another $1 billion for food under the overall U.N. emergency appeal.

"We estimate that 2.1 million people could require food aid over the next four weeks," WFP spokeswoman Christiane Berthiaume said. "If the war last longer than four weeks, the entire population would need food aid ... ."

In Washington, the U.S. Department of Agriculture said Thursday it would release up to 600,000 tons of U.S. wheat.

Health officials warned that any major flood of refugees was likely to cause disease outbreaks, particularly among women and children, the disabled and the elderly.

"If there are major movements of people, because of the water and sanitation situation inside the country, there will almost certainly be problems with disease outbreaks. We can also anticipate disease outbreaks in refugee camps," World Health Organization (WHO) spokesman Iain Simpson said.

NO MAJOR REFUGEE FLOWS

But so far, the U.N. High Commissioner for Refugees (UNHCR) said no major refugee flows had been detected to any of Iraq's six neighbors -- Iran, Jordan, Kuwait, Saudi Arabia, Syria and Turkey -- which all had opened their borders.

The agency is braced for a possible 600,000 Iraqi refugees who might flee the U.S.-led attacks, and has relief supplies for 300,000 stocked in Turkey, Iran and Turkey.

Iran said Thursday it was bracing for up to 1.2 million refugees from Iraq and complained that international help was slow in coming. It said Iran would only allow in those Iraqis whose lives were deemed to be in danger.