India’s prime minister, Narendra Modi, has vowed to punish all those responsible for Tuesday’s attack in Kashmir, pursuing them “to the ends of the Earth”.

Twenty-six men were killed in the tourist hotspot of Pahalgam, in the deadliest attack on civilians in the contested Muslim-majority territory since 2000.

“I say to the whole world: India will identify, track and punish every terrorist and their backer,” Modi said, in his first speech since the Himalayan attack.

India accused Islamabad on Wednesday of supporting “cross-border terrorism” and downgraded ties with its neighbour with a series of diplomatic measures. Pakistan has denied any role in the attack.

On Wednesday evening, Pakistan’s top diplomat in Delhi, Saad Ahmad Warraich, the charge d’affaires at the Pakistan embassy, was summoned by India’s ministry of external affairs, according to a diplomatic source and local media reports.

India had already closed a key land border with Pakistan, suspended a water-sharing treaty, and barred Pakistani citizens from entering under a visa exemption scheme.

On Thursday, police in Kashmir published notices naming three suspected militants alleged to have been involved in the attack, and announced rewards for information leading to their arrest. Two of the three suspected militants are Pakistani nationals, according to the notices.

The late-night summoning of Pakistan’s top diplomat reflected India’s “anguish” over the attack, the official said. “We raised our concerns and formally notified the measures India has taken in the wake of the terror attack.”

According to the diplomatic source and local media, the Pakistani diplomat was informed that all defence advisers at the country’s mission in New Delhi had been declared persona non grata and were expected to leave within a week.

Modi, who was speaking in Bihar state to launch development projects, first led two minutes of silence in memory of those killed.

“I say this unequivocally: whoever has carried out this attack, and the ones who devised it, will be made to pay beyond their imagination,” he said, speaking in Hindi in front of a large crowd. “They will certainly pay. Whatever little land these terrorists have, it’s time to reduce it to dust. The willpower of 1.4 billion Indians will break the backbone of these terrorists.”

He finished his speech with rare comments in English, directing them to an audience abroad. “Terrorism will not go unpunished,” Modi said. “Every effort will be made to ensure that justice is done.”

Manhunt launched after 26 tourists killed by suspected militants in Kashmir – video

India also announced on Wednesday it would withdraw its defence attaches from Pakistan, reduce its mission staff in Islamabad from 55 to 30 and declare Pakistan’s defence personnel persona non grata.

Modi has called for an all-party meeting with opposition parties on Thursday, to brief them on the government’s response to the attack.

Kashmir has been divided between India and Pakistan since their independence in 1947, with both claiming the high-altitude territory in full but governing separate portions of it.

In Islamabad, the prime minister, Shehbaz Sharif, was scheduled to hold a meeting of the national security committee to discuss Pakistan’s response, the foreign minister, Ishaq Dar, posted on X.

The Indus water treaty, mediated by the World Bank, splits the Indus River and its tributaries between the neighbours and regulates the sharing of water. It had until now withstood wars between the neighbours.

India would hold the treaty in abeyance, the Indian foreign secretary, Vikram Misri, said.

Diplomatic ties between the two nuclear-armed rivals had been loose even before the latest measures were announced, after Pakistan had expelled India’s envoy and said it would not post its own high commissioner to Delhi when India revoked the semi-autonomous status of Kashmir in 2019.

On Wednesday, Indian security forces fanned out across Kashmir as the army and police launched a large-scale manhunt for the perpetrators of the attack.

Amid rapidly rising tensions in the Himalayan region, which has been riven by militant violence since the start of an anti-Indian insurgency in 1989, survivors said the militants had asked men they had rounded up to recite Islamic verses before executing those who could not.

A little-known militant group, the Kashmir Resistance, claimed responsibility for the attack. Posting on social media, it expressed discontent that more than 85,000 “outsiders” had been settled in the region, spurring a “demographic change”.

Tuesday’s attack is seen as a setback to what Modi and his Hindu nationalist Bharatiya Janata party have projected as a huge achievement in revoking the special status that Jammu and Kashmir enjoyed, and bringing peace and development to the long-troubled Muslim-majority region.

Reuters and Agence France-Presse contributed to this report