Kenzo Oshima, UN undersecretary general for humanitarian affairs, warned on Monday that relief agencies had only a "short window" of six weeks to supply Afghanistan with enough food and supplies to avert a humanitarian disaster before the country's severe winter.

Mr Oshima, on a mission to Pakistan to investigate what aid officials are describing as a "vast" humanitarian crisis, said he could not "speculate" on the effects of any US-led military action on the intensifying relief effort, which is now directed at more than a third of Afghanistan's population.

Mr Oshima said the UN's "urgent" priority was to supply food to an estimated 7.5m Afghans inside the country now defined by the organisation as "highly vulnerable", or dependent on UN food aid.

The US on Monday pledged $100m towards the UN's appeal for the estimated $584m cost of the relief effort.

The World Food Programme, meanwhile, is gearing up to ferry as much as 1,000 tonnes of food daily into Afghanistan before the winter freeze, following the apparently successful delivery of 200 tonnes of wheat trucked into Kabul from the north Pakistani town of Peshawar over the weekend.

WFP officials estimate that Afghanistan needs a minimum stockpile of 56,000 tonnes of food before mid-November, in effect requiring a rolling convoy of food trucks into the country from its neighbours.

The agency had been shipping 25,000 tonnes of food into Afghanistan a month before the crisis created by the terrorism attacks on September 11 interrupted supplies. The WFP estimates Afghanistan's current food stocks at just 15,000 tonnes.

Two more WFP convoys ferrying a further 200 tonnes of wheat will on Tuesday leave Quetta, on Pakistan's western border with Afghanistan, designed to test the security of two further routes to Herat and Kabul. A further 1,400 tonnes of wheat is making its way into the north of the country from Turkmenistan and Tajikistan.

In addition, the WFP has landed 265 tonnes of "high energy biscuits" in Pakistan, Iran and Uzbekistan to supply refugee camps should any military action prompt a fresh flight of refugees to Afghanistan's borders.

Officials of the United Nations High Commissioner for Refugees (UNHCR) in Pakistan said crowds of tens or thousands of refugees on the country's borders had melted away in recent days, apparently as fugitives from the cities had returned to the Afghan countryside or slipped into Pakistan across remoter parts of the country's 1,400km border.

But UN officials said they were braced to cater for at least 1.5m refugees from Afghanistan. The UNHCR has been scouting over 100 sites on the Pakistan border alone for possible emergency refugee camps.

Mr Oshima said he had urged General Pervez Musharraf, Pakistan's president, to consider re-opening the country's borders with Afghanistan which have been closed since September 11. But the UN envoy also said he "understood" Islamabad's desire to keep the border closed and was "satisfied" with Islamabad's assistance in the aid effort.

Mr Oshima also met the Taliban's representative in the Pakistan capital, urging his government to allow international UN workers to return to Afghanistan but with strict guarantees for their security. He said the request was received "positively" by the Taliban ambassador, but had merely been passed on to the Taliban leadership in Kandahar.