The BBC's Owen Bennett-Jones reports from Islamabad

The Pakistani Foreign Ministry said it summoned India's High Commissioner in Islamabad, Satish Chandar, in the early hours of Thursday morning and told him that Pakistan expected the Indian Government to desist from any irresponsible act.

Pakistan said it took the action after receiving what it called credible information that an attack was to be mounted before dawn on Pakistani nuclear installations.

The BBC's Owen Bennett-Jones on the possibility of a Pakistani test

A statement from Pakistan's Foreign Ministry said any attack on the country's nuclear installations would "receive a swift and massive retaliation with unforeseen consequences".

"We are fully prepared to meet any eventuality in our defence," it said.

'Reports baseless' - India

India's Foreign Ministry spokesman, K. C. Singh: "We have no desire to heighten the tension"

But India dismissed the reports as malicious and baseless. A spokesman for the Indian Foreign Ministry in Delhi categorically denied that India was getting prepared to attack Pakistan.

The BBC's Daniel Lak in Delhi: 'There's a genuine sense of surprise'

The BBC Islamabad correspondent says Western diplomats said they had not heard any suggestion of a pre-emptive strike by India. He says Pakistan may be preparing international opinion to justify carrying out a test of its own.

Pakistan test could be imminent

Tension has been mounting between the two countries since India conducted a series of nuclear tests earlier this month. The international community has strongly criticised India's actions, and urged Pakistan not to retaliate.

However, reports suggest that Pakistan is now ready to conduct a test of its own.

Pakistan's Foreign Minister, Gohar Ayub, said on Wednesday that preparations for a nuclear test have been completed and all that remains is for Prime Minister Nawaz Sharif to order the detonation.

According to US intelligence reports, Pakistan is close to conducting its own nuclear test in southwestern Baluchistan province.

But the President of Pakistan, Rafiq Tarar, repeated on Wednesday that Islamabad is yet to make a decision on whether it will detonate a nuclear device in response to India's tests.