In what is likely to be its sternest challenge yet, Pakistan's military is poised to launch a major offensive in the coming days against militants in the remote mountainous terrain of South Waziristan, long rumoured to be the hiding place of Osama bin Laden.

The new Pakistani Taliban chief Hakimullah Mehsud, left, seen here in Sararogha, South Waziristan, on Sunday. Mehsud has vowed to strike back at Pakistan and the US for the increasing number of drone attacks along the border with Afghanistan. (AP image) After months of preparation that has involved massing 28,000 troops near the tribal area on the Afghan border, and after weeks of air strikes designed to soften up militant positions, senior military sources in Islamabad told The Independent that the long-awaited operation was imminent. The US has also increased drone strikes in the region to target key figures.

The operation to take on the 10,000-strong Mehsud network, formerly headed by Baitullah Mehsud, comes amid heavy pressure on Pakistan from Washington to continue its fight against militants, many of whom are involved in cross-border attacks on Western troops in Afghanistan.

On the weekend, Pentagon officials who have been monitoring the plans to launch the operation suggested that preparations were complete: "We would assess that they have plenty of force to do the job right now," said one official, who declined to be identified.

Confirmation of Pakistan's readiness emerged as militants yesterday underscored their enduring ability to strike at high-profile targets when a suicide bomber killed five people and injured several others in an attack on a UN office in Islamabad. Witnesses said the bomber entered the offices of the World Food Programme (WFP) and set off the bomb, triggering chaos.

"There was a loud blast, a flash of light, and the windows shattered," said Dominique Frankefourt, the WFP's deputy country director. "I was on the first floor of the two-storey building. I told everyone to get out as quickly as possible. But when I came down to the ground floor, there were people lying on the floor who could not move."

Officials said that such attacks are likely to increase in the weeks ahead if the operation in South Waziristan proceeds as anticipated.

Previous operations this year to drive the Taliban from the Swat valley and nearby areas resulted in a series of "revenge" attacks, many of them launched by the Mehsud network. The Interior Minister, Rehman Malik, told Pakistan's parliament yesterday that "we should expect a few more" such attacks in the days ahead.

A major operation in South Waziristan would also probably lead to fresh problems for aid agencies if large numbers of people flee the region, as happened earlier this year when more than two million people left their homes in Swat. Hundreds of thousands of those who fled have yet to return. UN officials would not comment on whether they believed a military operation was imminent but admitted that some preparations had been made. The UN said last week that 80,000 people had already left the South Waziristan area since June and estimated that up to 170,000 could follow if the operation goes ahead.

Confronted by a faltering operation in Afghanistan and amid mounting doubt about its ability to achieve a military victory there, the assault on South Waziristan is considered critically important by the US administration of President Barack Obama.

Washington wishes to see Islamabad take on militants responsible for cross-border attacks on US and Nato troops and prevent Pakistan from being a safe haven for such fighters.

For a long time, the US has complained that while Pakistan - which has received billions of dollars in military aid since 11 September 2001 - was prepared to target militants responsible for attacks inside the country, it was less willing to pursue those whose primary battlefields were inside Afghanistan. Indeed, it is an open secret that elements within Pakistan still consider such militants to be strategic assets.

Such concerns will not have been eased by the news that the Pakistan army has renewed a non-aggression pact with Maulvi Nazir, a Taliban leader who earlier this year said he was joining forces with Baitullah Mehsud and Hafiz Gul Bahadur to target Western forces across the border and support the Afghan Taliban leader, Mullah Omar.

"You have a lot of guys in South Waziristan that Pakistan treats as assets," said Christine Fair, an analyst with the Washington-based Rand Corporation. "Maulvi Nazir is an enemy of the US but he is most certainly an asset of Pakistan."

Baitullah Mehsud was assassinated in a CIA-operated drone strike at the beginning of August. The South Waziristan operation will focus on what remains of his network.

Hakimullah Mehsud, the new leader of the Pakistan Taliban, reportedly appeared over the weekend alongside other militant leaders to vow revenge for Baitullah Mehsud's killing, ending speculation that he too had been killed.

Pakistan has been reluctant to return to South Waziristan after humiliating retreats in earlier operations. Ensuing peace deals allowed the militants to regroup and consolidate their grip on the historically inhospitable territory that was a headache for British commanders in colonial times.

The difficulties are compounded by the presence of well-trained foreign fighters, notably al-Qa'ida-affiliated Arabs and central Asians. Punjabi sectarian militants who fought in the Swat valley have also moved into the region.

Pakistan's army is hampered by a lack of counter-insurgency training and an operation in South Waziristan, with winter snows not far away, would probably lead to more army casualties than those suffered in Swat.

World's most wanted: Could he be hiding in Waziristan?

It has been almost eight years since the whereabouts of Osama bin Laden - the man George Bush once famously said he wanted "dead or alive" - were known with any certainty.

But ever since the al-Qa'ida leader slipped out of the cave complex at Tora Bora and walked across the border into Pakistan in December 2001, the tribal areas of South and North Waziristan have been identified as possible hiding places for him. He has many allies in the area, such as the father-and-son Haqqani network based in North Waziristan. And the strict Pashtun code of hospitality would oblige local tribal leaders to provide him with shelter. Many experts believe Bin Laden has long been supplanted as the day-to-day head of the terror network that he unleashed. However, he still represents an inspirational figure to many jihadis, and his capture would be a major symbolic boost for the West. That, of course, is assuming that Bin Laden is still alive. Many believe he is long dead.