"Pakistan's nuclear energy programme dates back to the 1950s, but it was the loss of East Pakistan (now Bangladesh) in a bloody war with India that probably triggered a political decision in January 1972 (just one month later) to begin a secret nuclear weapons programme," the Congressional report said.

The report on Pakistan's nuclear weapons programme was submitted to lawmakers last month by Congressional Research Service (CRS), a research wing of the US Congress, which regularly prepares reports for Congressmen.

"Deterring India's nuclear weapons and augmenting Pakistan's inferior conventional forces are widely believed to be the primary motivation for Islamabad's nuclear arsenal," it said.

Observers point to the peaceful nuclear explosion by India in 1974 as the pivotal moment which gave additional urgency to the programme, CRS said in its report dated May 15.

"Pakistan's path to the bomb was through uranium enrichment technology, mastered by the mid-1980s. Islamabad gained technology from many sources," it said.

This extensive assistance is reported to have included, among other things, uranium enrichment technology from Europe, blueprints for a small nuclear weapon and missile technology from China.

However, exactly when Pakistan produced a workable nuclear explosive device is unclear, the report observed.

A 1985 National Intelligence Council report stated that Pakistan "probably has a workable design for a nuclear explosive device" and was "probably... a year or two away from a capacity to produce enough" highly enriched uranium for such a device, it said.

"A 1993 National Security Council report to Congress stated that Islamabad's nuclear weapons efforts 'culminated with the capability to rapidly assemble a nuclear device if necessary by the end of the 1980s," the report said.

"In any case, President Bush's failure to certify in 1990 that Pakistan did not 'possess a nuclear explosive device' led to a cut-off in military and financial aid under the Pressler Amendment," the CRS said.

When India conducted nuclear weapon tests on May 12, 1998 in Pokhran, Pakistan responded two weeks later on May 28 and May 30 with six tests at the Chagai Hills test site in western Pakistan.

Test yields were about 10 kilotons and 5 kilotons, according to seismic analysis, the report said.