The UN refugee agency has appealed to the government of Pakistan to postpone Friday's planned closure of one of the country's largest Afghan refugee camps.
A number of border camps have already been closed
It says that it is "deeply concerned" over the closure of Jalozai camp in North West Frontier Province (NWFP).
The UN High Commission for Refugees (UNHCR) has warned that "tens of thousands" of Afghans are being pressured into leaving by Friday.
It says their exodus could lead to a humanitarian crisis this winter.
But the Pakistani government says that the "voluntary repatriation" of the refugees will continue unabated and that the Jalozai camp must be closed in accordance with Friday's deadline.
"All Afghan refugees have to leave Pakistan and go back to their motherland with dignity and honour," the Federal Secretary for States and Frontier Regions, Sajid Hussain Chattha, told Pakistan's official APP news agency.
"We want the maximum repatriation of refugees before the advent of the winter season," he said, pointing out that 389 Afghan families had voluntarily returned on Monday.
The Pakistani government says that some of the camps - mostly inhabited by people who have fled decades of fighting in Afghanistan - have been used as a safe haven by Taleban and al-Qaeda militants.
But the UN says that refugees in Jalozai - which has a population of more than 100,000 people - have been given a "very short deadline" to leave, and that it will be "impossible to manage a safe, voluntary and sustainable repatriation operation".
It says that under a tripartite agreement signed in Islamabad on 2 August, the governments of Afghanistan, Pakistan and the UNHCR agreed that repatriation should be "voluntary and gradual".
They also agreed that the 2007 closure of four refugee camps - Katchagarhai and Jalozai in NWFP, and Jungle Pir Alizai and Girdi Jungle in Balochistan Province - should take into account the "security conditions and absorption capacity" in Afghanistan.
Millions of Afghans fled Soviet invasion and civil war
In May there were clashes over the impending closure at Katchagarhai refugee camp near Peshawar. Shops and houses were destroyed. There were also clashes at the Pir Alizai camp in Balochistan.
The refugees were angered over attempts to make them return or go to another camp in Chitral, in the remote Hindu Kush mountain range, where winters are harsh and means of livelihood scarce.
Katchagarhai camp was closed at the end of July following several months of preparatory work. The UNHCR says that this "measured approach" contributed to a peaceful conclusion, in which almost 40,000 Afghans returned.
But it has warned that camp closures carried out late in the year result in "secondary internal displacement" with returnee families living in inadequate and makeshift shelters over the winter.
The UNHCR says that the closure of Jalozai should be suspended until 2008 in order to permit a more "dignified and controlled conclusion to the process".
"Such a strong humanitarian gesture ahead of the holy month of Ramadan and winter would underline once again the extraordinary generosity and hospitality of Pakistan towards the Afghan refugee population," a statement released by the agency said.
Correspondents say many refugees do not want to return because they do not have land, shelter or jobs in Afghanistan.
Some have lived all their lives in Pakistan.