Indian Prime Minister Narendra Modi vowed on Monday to respond strongly to any future "terrorist attack", and warned that New Delhi would not tolerate "nuclear blackmail" in the event of further conflict with Pakistan.

A weekend ceasefire appeared to be holding, after four days of intense jetfighter, missile, drone and artillery attacks -- the worst violence between the two nuclear-armed neighbours since 1999.

US President Donald Trump on Monday said American intervention had prevented a "bad nuclear war".

"We stopped a nuclear conflict... millions of people could have been killed," he told reporters at the White House.

Modi, in a televised address to the nation -- his first since hostilities began last Wednesday -- said Pakistan has chosen to attack rather than help it fight "terrorism".

"If another terrorist attack against India is carried out, a strong response will be given," he said.

The alarming spiral towards all-out war began before dawn on Wednesday, when India launched missile attacks destroying what it called "terrorist camps" in the Pakistan-administered part of Kashmir.

"If Pakistan wants to survive, it will have to destroy its terror infrastructure," Modi said.

The conflict followed an April 22 attack on tourists in Indian-administered Kashmir which killed 26 civilians.

India accused Pakistan of backing the attack, but Islamabad denied involvement. (AFP)