New Delhi: Pakistan appears slightly ahead of India in terms of the number of nuclear warheads, with China having double the quantity, but the Indian defence establishment believes its deterrence capability is "robust", designed to ensure "survivability" for retaliatory strikes and firmly on track for further modernisation.

Pakistan now has 140-150 nuclear warheads compared to India's 130-140, with China hovering around 280, as per the latest assessment of the Stockholm International Peace Research Institute (SIPRI), released on Monday.

The US and Russia are in a different league altogether with 6,450 and 6,850 nuclear warheads respectively, accounting for 92 per cent of the world's nuclear weapons.

Defence establishment sources here said India, confronted with the collusive threat from China and Pakistan, has no other option but to systematically build nuclear deterrence that is "credible" and capable of inflicting massive damage in a retaliatory strike to any first strike by an adversary.

"Number of warheads does not really matter. With a declared no-first use nuclear policy, India is keen to ensure survivability and credibility of our assets and NC3 (nuclear command, control and communication) systems for assured secondstrike capabilities," a source said.

Pakistan has deliberately kept its nuclear policy ambiguous to deter India from undertaking any conventional military action despite repeated provocations, even as it steps up development at the Khushabnulear complex with help from China.

"For India, nuclear weapons are not war-fighting weapons. But we need credible minimum deterrence, with the certainty of massive retaliation," the source said. It is estimated that India would like to achieve a stockpile of around 200 warheads in the next decade.