Five people suspected to members of al-Qaeda, including three of Arab descent, made the mistake of running over a pedestrian on the sidewalk in a remote Pakistani town in December.

It alerted authorities to the fact that al-Qaeda members had crossed from Afghanistan into Pakistan. Pakistan responded to concerns the terrorist network's members were taking refuge in the country by deploying up to 60,000 troops along its 1,500 mile border with Afghanistan.

Officials say the deployments are meant to guard at least 172 mountainous crossings that could be used to cross into Pakistan.

"The effect of these deployments is meant to be simply to make this border impregnable," a senior Pakistani official told the FT. "With such large deployments, it's now practically impossible to have large flows of (al-Qaeda and Taliban) suspects".

But others say the arrangements are not fool-proof, and that smaller groups dressed as herdsmen and labourers could be entering the country.

Officials admit that smaller groups seeking to enter Pakistan as refugees have indeed crossed over.

Pakistan, fomerly a long-time supporter of Afghanistan's former Taliban regime, is keen to avoid becoming a safe haven for fleeing suspects.

Thousands of Pakistanis who were arrested in Afghanistan after the Taliban regime fell remain stranded there, mainly because Islamabad wants them to be fully screened before accepting them back.

In part a response to Pakistan's concerns, Hamid Karzai, the leader of Afghanistan's interim government, said his government was looking to "screen out" good Pakistani prisoners from bad ones.

"The good ones will definitely come home and the bad ones are a matter for all of us to deal with in a manner that we find suitable," he said.

Pakistani officials hope Pakistanis with links to al-Qaeda will be dealt with by the US, either being prosecuted for war crimes or set free if innocent.

Despite such care, Pakistani analysts and western officials warn that parts of the country are still potential safe havens for al-Qaeda members and others fleeing Afghanistan.

Across the 'semi-autonomous' tribal belt along the Afghan border - an administrative zone where tribal chiefs make many of their own rules - the Pakistani authorities are still finding it difficult to clamp down.

General Pervez Musharraf's government has allowed US troops to pursue suspects to the Pakistani border, letting the Pakistani military pursue in that country, has helped to reduce the number of safe-haven seekers.

However, a senior official said: "The border arrangements have minimised the numbers but nobody can say with certainty if nobody from Al-Qaeda has in fact crossed over. This is a challenge where we can minimise the numbers but not eliminate them".

Western officials said that because Pakistan had backed the Taliban for years, there were links between militant groups in Afghanistan and their sympathisers in Pakistan.

Pakistani investigators working to probe connections between domestic and foreign groups, especially al-Qaeda, say their most difficult task is to look at financial arrangements. Osama bin Laden is suspected of having donated large sums to Pakistani groups.

However, tracing those donations is difficult, partly because One of the main difficulties stems from Pakistan's financial system. From 1990 until 1998, Pakistani banks received approximately $11bn in deposits in onshore foreign currency accounts, but depositors were not obliged to reveal the source of their funds.

"This gave an opportunity to all kinds of illicit funds to be deposited in banks," recalls a former banker. "You could walk up to your bank carrying a million dollars in your brief-case. The bank could take your money but you never had to reveal where you got it all from."

While Pakistan suspended these foreign currency accounts in 1998 after the country's first nuclear tests led to western economic sanctions, but the deposits remain protected from investigations on their sources. Bankers also warn that going through these accounts would be a colossal task.

The traffic accident which led to the arrest of five al-Qaeda members is seen as an exception rather than a norm.

A retired military officer says: "Unless there are lucky breaks here and there, no one could ever be certain about al-Qaeda being totally out of Pakistan. The tentacles between Afghanistan and Pakistan ran so deep, its hard to know if they have been fully cut, despite the government's support to the US".