Pakistan is preparing for an underground nuclear test that could take place as early as Sunday, American officials said Wednesday, citing clear signs from spy satellites, foreign agents and Pakistan's political leaders.

Diplomatic, military and intelligence officials said Pakistan could test a nuclear warhead sometime next week at a desert site in response to five tests that its regional rival, India, has conducted since Monday. It would be Pakistan's first test of a nuclear device and would add one more country to the list of nations with an open nuclear ability.Despite the certain and severe economic and political consequences for Pakistan, and the effect such a test is bound to have on the already soaring tensions in the region, "no one expects them to not have a test," an administration official said.

President Clinton hastily sent a high-level diplomatic team to Pakistan on Wednesday after a discouraging telephone conversation with Pakistan's prime minister, Nawaz Sharif. The president said he asked the prime minister "to resist the temptation to respond to an irresponsible act."

But "Sharif was not able to give that reassurance," said Karl Inderfurth, the assistant secretary of state for South Asian affairs. "He told the president that he was under tremendous pressure to respond."

Members of Congress joined Clinton in appealing to Pakistan to resist the urge to conduct its own nuclear tests.

"We have to think, what's China's response going to be? This could be the idea that the nuclear genie is out of the bottle, and that there's going to be an arms race, a nuclear arms race in southeast Asia," said Sen. Richard Shelby, R-Ala., chairman of the Senate Intelligence Committee."All we can do now is to try to persuade the Pakistanis not to do this," Shelby said on NBC's "Today" show.

U.S. policymakers emphasized that Pakistan, which has fought three wars with India over the past 50 years, would be subject to the same scorn and sanctions as India if it goes ahead with its tests - but would receive international acclaim with restraint.

The U.S. Export-Import Bank announced it would stop issuing new approvals of financing for U.S. exports to India. The bank estimated the sanctions will immediately affect about $500 million in U.S. exports.

Pakistan on Thursday dismissed the importance of the U.S. sanctions. "What India has done is (barely) short of a declaration of war," Foreign Minister Gohar Ayub told The Associated Press.

"Not only Pakistan's, but also the security of other countries in this region has been threatened," said Tariq Altaf, a foreign ministry spokesman. "Some sanctions have been announced which are pretty irrelevant."

Ayub said Pakistan's response "has to be a calculated choice . . . to see the pros and cons (but) our policy has been for a balance of power with India."

Ayub meets Friday with U.S. Deputy Secretary of State Strobe Talbott and Gen. Anthony Zinni, commander of the U.S. Central Command.

"They'll talk to us and we will listen," said Ayub.

But he said that Pakistan, which has been living with U.S. sanctions since 1990 - when Washington cut off $650 million worth of aid, believing it had nuclear weapons - is not worried about more sanctions should it decide to explode a device.

"We've gotten used to sanctions . . . it's nothing new," he said.

Shelby, who had called the surprise "a colossal failure of our nation's intelligence gathering," scheduled a closed-door hearing Thursday to hear from CIA Director George Tenet and Sandy Berger, Clinton's national security adviser.

And the Senate Foreign Relations Committee planned a second day of hearings on the sub-ject. On Wednesday, Sen. Jesse Helms, R-N.C., the committee chairman, called for "a careful, top-to-bottom review of the state of our own nuclear infrastructure."

Meanwhile, China accused India of developing nuclear weapons to dominate South Asia while trying to paint Beijing as the region's biggest threat.

China joined an international ban on testing, but only after conducting 45 nuclear tests, the last one less than two years ago. Rumors persist that China has helped Pakistan develop nuclear weapons and missiles.

India followed up its tests with a conditional offer to join the world's nuclear control regimes. India has refused to sign the Comprehensive Test Ban Treaty approved by the United Nations in 1996, arguing it gave an advantage to the five nations that had already tested and refined sophisticated nuclear weapons.

Even the Tibetan spiritual leader, the Dalai Lama, who lives in exile in India, lauded India for asserting itself.

"We have to make every effort for the elimination of nuclear weapons," he said during a visit to the United States. "However, the assumption of the concept that few nations are OK to possess nuclear weapons and the rest of the world should not - that's undemocratic."