India launched air strikes yesterday against a terrorist training camp hidden on a hilltop amid thick forests in Pakistan, in what it called a pre-emptive strike to prevent further terror attacks.

It was the worst escalation in tensions between the two nuclear-armed rivals since 2001.

Indian officials said the training camp belonged to Jaish-e-Mohammed (JeM), a Pakistan-based terror group that claimed credit for the Feb 14 suicide bombing in which 40 paramilitary soldiers were killed in Pulwama district in Kashmir.

India had blamed Pakistan for having a "direct role" in the attack and had warned of reprisals.

In the air assault, 12 Mirage 2000 single-engine fighter jets dropped bombs on the camp, reported ANI news agency.

Indian Foreign Secretary Vijay Gokhale said the air strike was a pre-emptive attack on a non-military installation.

It was conducted because India had received intelligence that suicide bombers were being trained in the camp in Balakot town, Khyber Pakhtunkhwa province, more than 50km from the Line of Control (LoC), the de facto border between the two countries.

Mr Gokhale said the camp was run by Maulana Yousuf Azhar, the brother-in-law of JeM founder Masood Azhar. "Credible intelligence was received that JeM was attempting another suicide terror attack... and (militants) were being trained for this purpose. In the face of imminent danger, a pre-emptive strike became absolutely necessary,'' he said.

Mr Gokhale added that "a very large number" of JeM terrorists, trainers, senior commanders and militants were killed.

Indian media reported up to 300 militants were dead, citing government sources. Pakistan, however, denied there was any casualty or damage. Military spokesman Asif Ghafoor also claimed that the air strikes had hit open land.

"Under forced hasty withdrawal, (Indian) aircraft released payload which had free fall in open area. No infrastructure got hit, no casualties,'' he tweeted, revealing that Pakistan had scrambled its jets.

Pakistani Foreign Minister Shah Mehmood Qureshi said: "India has violated the LoC and Pakistan reserves the right to give a befitting response."

The country's National Security Committee said it "strongly rejected Indian claims of targeting an alleged terrorist camp near Balakot and the claim of heavy casualties."

It warned that "Pakistan shall respond at the time and place of its choosing" to India's aggression.

Pakistan also accused India of violating the ceasefire signed in 2003.

The two countries have fought three wars over Kashmir and regularly exchange fire across the de facto border in spite of the ceasefire.

India has for years demanded action against Pakistan-based terror groups like JeM and Lashkar-e-Taiba which have carried out multiple attacks in India, including the 2008 Mumbai terror attacks which killed 166 people.

In 2016, India carried out surgical strikes on terrorist launch pads along the border in retaliation for an attack on an Indian army base in Uri in Kashmir that killed 19 soldiers.

Within India, the latest air strikes gained support across an otherwise deeply divided polity. Opposition Congress' president Rahul Gandhi tweeted: "I salute the pilots of the IAF (Indian Air Force)."

Former Jammu and Kashmir chief minister Omar Abdullah tweeted: "The post-Uri strike was to avenge our losses, Balakot was a pre-emptive strike to prevent an imminent JeM attack. Totally new ball game.''

With elections scheduled to take place before May, Prime Minister Narendra Modi had been under pressure to take some kind of military action following the Pulwama attack. At a rally in Rajasthan yesterday after the air strikes, he said: "I want to assure you, India is in safe hands.''

For now, speculation is mounting within India over how Pakistan may retaliate.

Said Air Vice-Marshal (Retired) Manmohan Bahadur: "They have to decide how much they want to escalate. Do they want to go to war?''