The US is bitter at Pakistan's defiance of President Clinton's appeals not to respond to India's nuclear tests with its own explosions. Even the threat of US economic sanctions and offers of aid were ignored in a further blow to Washington's foreign policy.
Mr Clinton is said to have spoken three times by telephone to the Pakistani President, Mr Nawaz Sharif, the last time as recently as Wednesday night. A Pakistani delegation was due in Washington today for further talks with US officials.
President Clinton was quick to react to the news of the Pakistan nuclear tests: "First, I too deplore the decision. By failing to exercise restraint in responding to an Indian test, Pakistan lost a truly priceless opportunity to strength en its own security, to improve its political standing in the eyes of the world. And although Pakistan was not the first to test, two wrongs don't make a right."
The President said the US now has no choice but to impose sanctions as is required by law. Similar sanctions have been applied to India following its nuclear tests earlier this month.
"Now, I want to say again - it is now more urgent than it was yesterday that both Pakistan and India renounce further tests, sign the Comprehensive Test Ban Treaty, and take decisive steps to reduce tensions in South Asia and reverse the dangerous arms race.
"I cannot believe that we are about to start the 21st century by having the Indian sub-continent repeat the worst mistakes of the 20th century when we know it is not necessary to peace, to security, to prosperity, to national greatness or personal fulfilment.
"And I hope that the determined efforts of the United States and our allies will be successful in helping the parties, who must themselves decide how to define their future, to defuse the tensions and avoid further errors," Mr Clinton said.
The US had been expecting Pakistan to go ahead with nuclear tests as its intelligence services in recent days reported preparations at the Chaghi underground test site. Unlike the Indian tests, which caught the US by surprise, this time the intelligence services were alerted and were telling the President that the explosions could be expected at any moment.
Since the Indian tests, the US has been making desperate efforts to head off the Pakistan response. But its position was weakened by the fact that the US has been applying military sanctions to Pakistan since 1990 when it became clear it was embarked on a covert nuclear programme.
Thus, the US refused to deliver 28 F-16 fighter aircraft which Pakistan had paid for. The US was believed in the past weeks to be offering to deliver the aircraft or return the payments if Pakistan would refrain from nuclear tests.
Now the US will have to apply economic sanctions to Pakistan and halt all military aid. This means that the US will use its influence in international organisations such as the International Monetary Fund and the World Bank to halt future aid and loan packages which could total $1 billion.
This week, the US persuaded the World Bank to delay indefinitely three loans to India worth $855 million. Pakistan's struggling economy would be even more vulnerable than India's to the halting of international aid.
Pakistan, one-quarter India's size, is more dependent on foreign aid and has smaller foreign reserves. "Pakistan is much more vulnerable to sanctions than India," Mr Paul Kreisberg, a former US diplomat and senior fellow at the Woodrow Wilson Center, said. "This could collapse the entire Pakistani economy." Pakistan's large proportion of non-performing bank loans and dependence upon money from the World Bank and IMF mean strict sanctions could cause the economy to "grind to a halt," he said. "That could create a lot of civil unrest, undermine the government's credibility, and favour the most radical elements in society," Kreisberg said, adding that this scenario might induce Congress to soften the nuclear penalties. somewhat.
The law penalises any country other than the five declared nuclear powers - Britain, China, France, Russia, and the US - that detonates a nuclear device. Though humanitarian aid is exempt, the law mandates an end to US economic assistance, valued at some $4 million a year since Pakistan first came under US non-proliferation sanctions in 1990. It also requires a freeze in commercial arms purchases valued at $80 million to $120 million dollars a year.
The White House spokesman, Mr Mike McCurry, said the president now has "enormous concerns" about the escalation of hostilities between the two countries. The tests make Pakistan the first Islamic nuclear power.