BBC News' Owen Bennett-Jones: 'Pakistani people told to prepare for austerity'

The officials, who would not be identified, said American spy satellites monitoring a site in Pakistan had picked up signs of preparations; one official said a sixth nuclear test could be readied in a couple of days.

They gave no further details.

The news came just hours after Pakistan declared a state of emergency following its underground nuclear tests on Thursday.

The Pakistani state-run news agency said the President of Pakistan Rafiq Tarar cited a threat by "external aggression" to the security of Pakistan.

The president did not identify who the aggressor was, but Pakistan has accused neighbouring India of threatening to attack its nuclear installations.

India has called the allegations "baseless and malicious".

BBC correspondent Ben Brown reports on the nuclear rivalry on the subcontinent

The BBC correspondent in Islamabad says the decision to declare a state of emergency may be more about limiting damage to Pakistan's economy from sanctions.

The state of emergency suspends the country's constitution and legal system as the country prepares to face massive international sanctions.

The move allows the government to take on new powers, suspending the licences it grants to money-changers, meaning that all foreign currency transactions will have to be at the official rate.

Tests 'deplored'

The Indian Prime Minister, Atal Behari Vajpayee, said Pakistan's nuclear tests vindicated India's decision to conduct tests of its own earlier this month.

In the rest of the world, there was near universal condemnation of Pakistan.

The tests were carried out around Chagai about 500km from Islamabad

President Clinton said: "I deplore the decision. By failing to exercise restraint Pakistan has lost a truly priceless opportunity ... two wrongs don't make a right."

He said that America would impose sanctions on Pakistan in the way it imposed them on India as punishment for the nuclear tests it conducted earlier in the month.

President Clinton: Pakistan has lost a 'priceless opportunity'

"It is now more urgent than it was yesterday that both India and Pakistan renounce further tests and sign the comprehensive test ban treaty ... and reverse the dangerous arms race," he said.

Describing the tests as "extremely deplorable" Japanese officials have decided to impose economic sanctions on Pakistan as strict as those imposed in India.

Japan is Pakistan's biggest aid donor providing 32bn yen in loans and 5.7bn yen in grants-in-aid in 1997.

Sanctions are also expected to entail the suspension of new grants and loans, but will not include emergency assistance and humanitarian support.

Pakistan 'forced to carry out tests'

Pakistan's Prime Minister Nawaz Sharif said that Pakistan had been forced to carry out the tests.

Nawaz Sharif: "We could have done this years ago"

He said that the series of five controversial tests by India earlier in the month had made the action "inevitable".

"We never wanted to participate in this nuclear race. We have proved to the world that we would not accept what was dictated to us."

Nawaz Sharif: "We could not have remained complacent."

"India exploded nuclear devices. Today we've paid them back," he added.