Telephone calls Monday threatening rocket attacks on the British high commission triggered the closures by both missions, according to Pakistani officials who would not be named.

Representatives of the British and US missions would not comment on the nature of the threat.

"We've taken the decision to remain closed today (Tuesday) for staff safety considerations," British high commission spokesman Tim Handley told AFP.

"We're liaising closely with the relevant authorities and we're monitoring the situation very closely."

The US embassy cancelled a Fourth of July celebration planned for Monday, when embassy staff also happened to be observing a public holiday.

It decided to remain closed Tuesday because of "a rather serious threat to the diplomatic enclave against a specific embassy," a US spokesman told AFP, without elaborating.

The enclave houses most of Islamabad's foreign missions on the capital's southeast outskirts.

More than 1,000 police are deployed to guard the area as part of a special diplomatic protection unit within the police force.

The British and US missions are among the most heavily fortified, surrounded by razor wire, fortified gates, several police-manned checkposts, and guarded by paramilitaries camped permanently in trees opposite the buildings.