U.N. officials are criticizing Iran for pressuring Afghan refugees to return home against their will. The situation could aggravate Afghanistan's difficulties in repatriating more than a million-and-a-half people.

A spokeswoman in Kabul for the United Nations High Commissioner for Refugees, Maki Shinohara, said the relief agency is bracing for a flood of returnees from Iran. "UNHCR is alarmed at the sudden increase in the number of refugee returns from Iran, which we believe is a result of induced pressure by the authorities," she said.

Ms. Shinohara said the agency's office in Mashad, Iran, has warned that three-thousand people are crossing the border at Dogharoun. She said UNHCR staff in the western Afghan city of Herat are scrambling to erect 100 additional tents to house up to 500 of the new arrivals.

Ms. Shinohara said many returnees report they are pressured to leave Iran. Some say their children are denied registration for the upcoming school year. And those refugees without proper documentation say they are told to obtain exit visas.

She said about 10,000 Afghans are returning each week now, compared with 6,500 per week last month, and many may eventually end up back in Iran. "UNHCR warns that premature, forced or induced returns at this time will not be sustainable, and may lead to a reversal of movements in the future," she said.

U.N. agencies are already grappling with that is described as the biggest repatriation of people in history. This year more than a million-and-half Afghans have returned home, mostly from Pakistan and Iran.

Foreign aid experts say many of the returnees will need assistance for at least the next two or three years to rebuild lives that have been shattered by 23 years of conflict in Afghanistan.