"These tests can only serve to increase tensions in an already volatile region," Mr. Clinton said in a statement.
"Both India and Pakistan need to renounce further nuclear and missile testing immediately and take decisive steps to reverse this dangerous arms race," he said.
U.S. intelligence confirmed that Pakistan tested a nuclear device Saturday but said its explosive yield was smaller than claimed and reported no signs that either India or Pakistan plan further weapons tests.
"Our information points to more like two kilotons and a single blast," said a U.S. official Saturday. Reports out of Pakistan were that the weapon was 18 kilotons, or almost the same strength as the weapon the United States dropped on Hiroshima in 1945.
The Clinton administration, trying to prevent a "horrifying conflict" between two new nuclear powers, is enlisting leading member states of the United Nations to pressure the feuding South Asian neighbors to stop atomic weapons development.
Secretary of State Madeleine Albright was arranging a meeting next week among foreign ministers of the five permanent Security Council members to discuss the crisis, State Department spokesman James P. Rubin said Friday.
The Council - whose five permanent members are the United States, Russia, China, Britain and France - expressed concern Friday over "peace and stability" in the region. The body also deplored Pakistan's tests - as it did India's two weeks ago - and called on both nations to sign nonproliferation and nuclear test ban treaties.
Rubin acknowledged U.S. and international influence over the rival neighbors is "very limited." India and Pakistan have fought three wars, including two over the northern territory of Kashmir, and appear headed toward an open nuclear arms race.
U.S. spy satellites captured final preparations for Saturday's test and its aftermath at a remote site in western Pakistan, about 100 miles from the Chagai Hills site where Pakistan reported conducting five nuclear tests Thursday.
"They have said this completes their testing. For the moment, we don't have any reason to disbelieve them," the U.S. official said. He made the same observation about India.
President Clinton and U.S. officials, through telephone calls and personal diplomacy, were unable to stop the nuclear tests.
Mr. Clinton imposed stiff sanctions on India, cutting all but humanitarian aid and imperiling international loans. Sanctions the Mr. Clinton administration must impose on Pakistan under U.S. law will block any renewal of U.S. military and economic aid and jeopardize a $1.6 billion loan from the International Monetary Fund.
As a condition for lifting sanction, the president said the two countries should sign the Comprehensive Test Ban Treaty (CTBT) and turn their backs on nuclear weapons programs.
Senate Majority Leader Trent Lott, R-Miss., said he supports sanctions, but criticized the administration's push for the test ban treaty, which hasn't been ratified by the Senate.
"The CTBT will not enter into force unless 44 countries, including India and Pakistan ratify it. That is not likely," he said.
Written by Laura Myers