Rumours of a US attack on Bin Laden are circulating in Pakistan and a Gulf paper reported that an American operation would be imminent if the types of explosive used in East Africa matched those used in other attacks linked to Bin Laden.

American ambassador to Pakistan, Thomas Simons, also denied that such an operation was on the cards. He dismissed such a possibility while talking to newsmen in Islamabad after seeing of about 200American embassy staff and their dependents at Islamabad airport.

The Americans had arrived in a convoy on Tuesday afternoon at the Islamabad airport, in four buses accompanied by the Ambassador's car and four motorcycle riders withsirens blaring, provided by the Pakistan government.

The US nationals were accompanied by the Ambassador to a chartered plane which later took off for Brussels.

But there were few non-diplomatic staffers amongst the passengers as most Americans not associated with the embassy have decided not to evacuate.Simons said "People are thinking about leaving but there is no general rush for the exits."

He added that those who had been evacuated would also return soon as the embassy was only following procedure, which he informed was also being followed in some other diplomatic posts in other parts of the world where a threat was perceived.

"This is not a lack of faith in the Pakistan government," said an American embassy spokesman who took pains to point out that the measure was precautionary.

Some American diplomats remained behind as their families, non-essential staff and their relatives departed after the US State Department ordered their departure after the August 7 bombing of US embassies in Nairobiand Dar-es-Salaam. The American ambassador will also stay behind along with other key diplomats.

Though ordinary Americans have been strongly advised to leave as well, few took up their government's offer of a ride on an American charter plane. Americans not connected with the embassy were briefed by the ambassador on Monday evening and many said that despite the seriousness of the warning, they were inclined to stay for the time being as there had been no specific threat against American nationals in Pakistan.