About 40,000 Pakistanis are on the move even before a military offensive begins in the Taliban stronghold of South Waziristan, UN officials said today, and are headed for communities already stretched to the limit.

Nearly two million people have fled fighting in northwest Pakistan, most since early May when the military began an offensive against Taliban insurgents, prompting the United Nations to launch an appeal for $543 million in aid to avert a long-term humanitarian crisis.

About 35 per cent of that figure has been reached, UN special humanitarian envoy Abdul Aziz Arrukban told Reuters, but the target has taken on a new urgency now that many thousands more displaced can be expected from South Waziristan.

"It should be more, it should be bigger than that number but I believe some countries are working on donations now and hopefully we will get it fairly soon," said Mr Arrukban, UN secretary-general Ban Ki-moon's humanitarian envoy since 2007.

A Taliban thrust into northwestern Buner district in early April raised fears about the future of nuclear-armed Pakistan, a vital ally for the United States in its battle to defeat al Qaeda and its allies and to stabilise neighbouring Pakistan.

The military responded later that month and its main offensive, welcomed in Washington after doubts about Islamabad's commitment to the fight against militancy, began in earnest in early May in the scenic Swat valley, once a tourist attraction.

Fighter jets have hit targets in South Waziristan in recent days ahead of the latest phase of the offensive, in which the military plans to target Pakistani Taliban leader Baitullah Mehsud in his stronghold on the Afghan border.

About 37,000 people had already left their homes in South Waziristan, said Manuel Bessler, head of the UN Office for the Coordination of Humanitarian Affairs, citing military figures.

Mr Bessler said Pakistan presented a unique problem for humanitarian officials because 80 per cent of the displaced were not in camps set up by the United Nations and other agencies but were staying with family and friends in "host" communities.

"Their capacity is stretched, if not to say over-stretched," Mr Bessler said of the host communities, some of which have been sheltering the displaced since last August.

"It's very different to the displaced in Africa, where most are in camps," he told Reuters.

Pakistan is being kept afloat by a $7.6 billion International Monetary Fund loan, underscoring the need for outside help for the displaced. Arrukban said UN aid operations in Pakistan cost about $2 million a day.

He said he would travel soon to some Gulf Cooperation Council states and other Middle Eastern countries on Ban's behalf in search of aid. The coming monsoon added more urgency, he said.

Mr Arrukban described the scale and speed of the displacement in Pakistan as "unprecedented". He hoped many would be able to return home soon but would not comment directly on whether security and conditions were right yet for that to happen.

He said food supplies were in good shape but more medical services, tents, sanitation and water supplies were needed in host communities and the 45 "hubs" set up for the displaced.

About half of Buner's 700,000 population fled the fighting but have started trickling back as security improves. Roads into Buner from a camp in nearby Mardan were packed at the weekend as about 6,000 people returned, Reuters witnesses said.

Mr Bessler said the fact that many of those from rugged, mountainous South Waziristan had second homes they used to escape the harsh winter might help ease some of the added strain.

He said he was aware of reports that some Pakistanis, mainly ethnic Pashtuns, had fled across the border into Afghanistan, itself devastated by 30 years of war, to escape the fighting.