The source told AFP that the deal had been in the works for years and that Saudi Arabia expected Pakistan's arch-rival India to understand the kingdom's security needs.

When asked if the deal meant Pakistan's nuclear weapons could be used to defend Saudi Arabia, Ali Shihabi, an analyst close to the royal court, told AFP: "Yes, it does".

"Nuclear is integral to this deal and Pakistan remembers that the kingdom effectively financed their nuclear programme and supported them when they were sanctioned," he added.

"India will understand the security needs of Saudi Arabia. Saudi maintains excellent ties with India."

Pakistan's Defence Minister Khawaja Asif reportedly also told a local broadcaster that the country's nuclear programme would be made available to Saudi Arabia if needed, following the signing of the agreement.

The mutual defence agreement was signed in Riyadh just days after an Israeli strike targeted Hamas leaders in neighbouring Qatar that sent shock waves through the Gulf states, who have long depended on the United States for their security.

The new pact also comes mere months after Pakistan and India engaged in an intense four-day conflict in May that killed more than 70 people on both sides in missile, drone and artillery fire, the worst clashes between the nuclear-armed neighbours since 1999.

India's Prime Minister Narendra Modi was on a diplomatic visit to Saudi Arabia in April when he cut short his trip following a deadly shooting attack on tourists in India that sparked the conflict.

India and Pakistan have long accused each other of backing militant forces to destabilise one another.

Saudi Arabia is believed to have played a key role in defusing the conflict.

The kingdom has been a key supplier of oil to India, the world's most populous country, for years.

India's rapidly developing economy relies heavily on petroleum imports, with Saudi Arabia ranked as its third-largest supplier according to the Indian foreign ministry.

Islamabad has also fostered close links with Riyadh for decades, with over 2.5 million of its nationals estimated to be living and working in the kingdom.

Saudi Arabia has long been a bulwark of economic support for Pakistan's faltering economy.