The American Secretary of State, Madeleine Albright, has called for a ministerial-level meeting of the permanent members of the Security Council - Britain, France, China, Russia, and the US - to find ways to persuade both countries to sign international nuclear test-ban treaties.

Madeleine Albright : wants top-level talks

President Clinton has sent a strongly-worded message to India and Pakistan, warning of serious consequences for their relationship with the United States if they take any new steps towards building a nuclear arsenal.

But American intelligence officials say Pakistan may be preparing to do as India did and hold a second round of nuclear tests, perhaps even this weekend.

Owen Bennett-Jones in Islamabad: first details of tests

Pakistan's leading nuclear scientist, Dr Abdul Qadeer Khan has said the biggest of the devices exploded in the underground tests on Thursday was more than 30 kilotonnes - nearly twice the size of the bomb dropped by the United States on Hiroshima in 1945.

Pakistan is also expected to test a medium-range Ghauri missile in the next few days, underlining the message to India that Pakistan can not only to make nuclear bombs, but also launch them.

The BBC state department correspondent says American officials admit the international community has shown itself to have limited influence with India and Pakistan.

But Washington is determined to halt a potential nuclear arms race, and is also looking at ways of improving communication between the two countries.

Earlier, the United Nations Security Council strongly condemned the tests carried out by Pakistan and urged the Asian nations to cease their programmes.

"A strong message": US ambassador Bill Richardson

A statement called on the two countries to sign the nuclear non-proliferation and test ban treaties. It said they should also resume talks.

The United States ambassador to the UN, Bill Richardson, said it was critically important that tensions in South Asia were reduced.