The expected exodus of refugees from Iraq has yet to materialize, U.N. officials said Friday, but cautioned that it may be too early in the 2-day-old U.S.-led invasion to tell how bad things may become.

International relief agencies predicted that a large-scale humanitarian crisis could develop inside Iraq, however, because of shortages of food, health care and other basic needs in the coming days and weeks.

"It is clear that Iraq is on the brink of an unprecedented humanitarian crisis and that UNICEF is facing possibly the largest and most complex humanitarian operation we've ever undertaken," United Nations Children's Fund spokeswoman Wivina Belmonte said in Geneva.

Military operations in the western Iraqi desert may be one reason few people are venturing toward the Iraq-Jordan border, said Peter Kessler, spokesman for the U.N. High Commissioner for Refugees.

He cited an unconfirmed report of a bridge bombed along the lone westbound highway through the desert as possibly hindering westward movement.

The U.N. agency had no reports of Iraqi refugees crossing the Jordanian, Syrian, Turkish or Iranian frontiers.

However, thousands of Iraqi Muslim Shiites have crossed into Syria in the past four weeks, ostensibly as pilgrims, said Syrian government officials, speaking on condition of anonymity.

The Iraqis came to pay homage at Shiite shrines for the feast of Ashoura last week, but many decided to stay on, saying they would return to Iraq only when the war ends.

In Amman, refugee officials said that almost 500 third-country nationals, most of them workers or students from Sudan, had entered Jordan from Iraq since Wednesday.

A group of 140 was scheduled to fly home to Sudan on a chartered plane, said Chris Lom, spokesman for the Geneva-based International Organization for Migration.

UNICEF, meanwhile, said the dangerous situation inside Iraq kept it from getting two truckloads of food to hundreds of handicapped and orphaned children in the Iraqi capital on Friday.

"In 1991, many children in institutions died," UNICEF spokesman Geoff Keele said in Amman, referring to the 1991 Gulf War.

Aid officials say millions of Iraqis, most of them children, may soon face food shortages because of the collapse of the U.N.-supervised, government-operated food rationing system in Iraq.