Trapped inside borders with relief workers gone, supplies dwindling fast and war looming, Afghans who are among the world's toughest survivors now face potential death in large numbers.

"It is impossible to overestimate just how bad it is," said Rupert Colville of the U.N. refugee agency. "It's almost inconceivable, and nobody can get in to film or describe it."

No matter what happens, aid officials added Thursday, events in Afghanistan could fast deteriorate into one of the worst humanitarian crises ever.

Already, Colville and others say, the scale threatens to approach the Rwandan catastrophe or the worst days of Bosnia. Winter is weeks away, and border crossings are closed. Relief efforts are all but paralyzed.

In the most remote areas, aid workers report, conditions approach famine and are deteriorating by the day. Hundreds of thousands of Afghans are believed massed along the borders, trying to escape. Afghanistan's neighbors have refused to let them cross.

"Millions are in extraordinarily bad shape," Colville said by telephone from the Pakistani city of Quetta. "They are not Taliban, not fighters, not friends of Usama bin Laden. Just ordinary Afghans."

Many people are caught in destitute hamlets up to a week's donkey ride from the nearest lean-to shop. "I've seen Afghans who didn't know what a doctor was, let alone had ever seen one," Colville said.

If airstrikes or military incursions start, already weakened civilians will have to make their way to distant relief centers. Many are too old or sick to move and those who stay with them may die.

Aid workers note that in the Balkans and Rwanda, calamity came quickly and many people managed to escape to refugee camps from almost normal daily lives. In Afghanistan, it is different.

"These people have been crippled by 23 years of conflict, a decade of neglect by the international community and four years of devastating drought," Colville said.

Mike Sackett, coordinator of U.N. operations in Afghanistan, reached in Islamabad before flying to Iran to press for more help, made a similarly grim assessment.

"It was already a terrible crisis and now it is worse," he said.

Huge numbers of exhausted refugees are on the road. Some have fled cities for rural areas. Others are trying to leave the country. Large numbers are weak and vulnerable from years of hardship.

"These aren't well-fed Bosnians with 20 or 30 pounds of spare body weight," said Peter Kessler, U.N. refugee agency spokesman in Islamabad. He said 7 million of the estimated 21 million people in Afghanistan need aid.

U.N. Secretary-General Kofi Annan said Thursday the United Nations will need more than a half-billion dollars for air drops and other aid for the refugees if the United States attacks, and he appealed to donor countries to help raise the money.

Fear of war forced all expatriate U.N. staff and voluntary aid workers to flee Afghanistan, leaving a local staff with dwindling stocks.

U.N. officials say no one is sure how many Afghans are refugees.

Officially, about 4 million refugees are outside Afghanistan, with at least 1 million — now perhaps many more — displaced within the country. About 4.7 million returned after the Soviet invaders departed in 1989, U.N. figures show, but some of those have left again.

"We already have a crisis, whatever happens, and every day we are losing time — this is critical, critical, critical," said Stephanie Bunker, spokeswoman for the U.N. Humanitarian Coordinator in Afghanistan.

Afghan society is built around shared hard times, allowing them to live through almost anything in their high mountain reaches and broad deserts.

"If someone has one cup of tea, he'll give you half," Bunker said. "It's their way. Now this is breaking down. Many have nothing. The grip on survival itself is slipping. Quite possibly, people will starve to death."

Donor support of $330 million for 2001 covers only the most basic needs of the weakest, she said, adding that a well-funded relief program in Afghanistan would run into the billions each year.

Fresh supplies trickle in from Iran through two northern points that are far from the Taliban stronghold at Kandahar in the south, near where bin Laden has made his base. If Western allies attack near Kandahar, civilians in the area would have no access to aid.

The U.N. High Commissioner for Refugees appealed Wednesday for $269 million for 1 million people expected to flee across borders in case of hostilities.

"When food aid can't get in, people will have to move," Colville said. "No option. There are millions we don't know about, not to mention the hundreds of thousands leaving cities in the past two weeks."

Russia's border guard chief Col. Gen. Konstantin Totsky estimates that 120,000 Afghans are massed on Afghanistan's northern border and U.S. airstrikes may push them across to poor former Soviet republics that lack the resources to care for them.

Two years ago, Colville wrote a rueful column in the International Herald Tribune, taking care to note it was his personal opinion and not a U.N. statement.

He said the Taliban killed 5,000 to 8,000 people over four days in Mazar-e-Sharif, near the Soviet border, but the atrocity went essentially unreported. While people in Western countries cared about the Balkans, he wrote, Afghanistan was left to its own fate.

Back in his role as a U.N. spokesman, he is more measured but still clear in warning about an approaching calamity.

"The world can't just allow Afghanistan to rot," he said. "What happened at Mazar-e-Sharif was very symptomatic of the outside world's neglect." Such neglect, he concluded, "is possibly a genocidal act."