India announced Monday that it had conducted three underground nuclear tests - its first in 24 years - in the desert state of Rajasthan, close to the border with Pakistan.

India's archenemy Pakistan condemned the experimental blasts and said they would suck Pakistan into an arms race. Islamabad asked the international community to condemn them and vowed to make its defenses "impregnable against any Indian threat, be it nuclear or conventional."The tests stunned much of Washington and may force the United States to impose sanctions, U.S. officials said.

Indian Prime Minister Atal Behari Vajpayee told a hurriedly summoned news conference that the controlled blasts were carried out at 3:15 p.m. (4:15 a.m. MDT) with a fission device, a low-yield device and a thermonuclear device.

"The measured yields are in line with expected values. Measurements have also confirmed that there was no release of radioactivity into the atmosphere," he said in a statement from the lawn of his residence, a national flag standing beside him.

The British Geological Survey said its equipment had picked up tremors from the unexpected tests measuring 4.7 on the Richter scale - the equivalent of a light earthquake.

The tests, India's first since its only previous test in 1974, come less than two months after the coalition government led by Vajpayee's Hindu nationalist Bharatiya Janata Party (BJP) took power.

The BJP made the option to introduce nuclear weapons a key plank of its platform in the elections.

The government said last month that it would decide whether to build nuclear weapons after a strategic defense review.

India has refused to sign the Nuclear Non-Proliferation Treaty and the Comprehensive Test Ban Treaty, saying they are discriminatory because they allow a few countries to indefinitely hold nuclear arms with no commitment to disarm, while forcing all others to relinquish nuclear weapons.

The nuclear tests follow a spate of controversial comments by India's outspoken defense minister, George Fernandes, on the military threat posed by China, India's nuclear-armed neighbor to the north.

India and China fought a brief but bloody war in 1962 - two years before China held its first nuclear test. Many Indian analysts say that test spurred New Delhi's test a decade later.

Fernandes also reacted sharply last month to an announcement by Pakistan, which has been to war with India three times, that it had test-flown a long-range missile.

He accused China of supplying Pakistan with the missile technology and said India's Prithvi missile could reach anywhere in Pakistan.

Pakistan says it is capable of producing nuclear weapons but has never conducted a test.

Indian experts gave the unexpected tests a warm welcome.