The Pakistan-based Jaish-e-Mohammad (JeM) claimed responsibility for the horror and released a video clip of the suicide bomber, a 'commander' identified as Adil Ahmad Dar, which it claimed was shot before the young man carried out the strike in Lethpora, about 30 km from here.

While precise details of the attack were still unclear, police sources said the suicide bomber-driven SUV came along the Central Reserve Police Force (CRPF) bus when a 78-vehicle convoy carrying 2,547 security personnel was going from the transit camp in Jammu to Srinagar and rammed it into the bus around 3.15 p.m., triggering a deafening explosion.

So devastating was the attack -- one report spoke of 200 kg of explosives, including probably RDX, packed into the SUV -- that the CRPF bus was left a mangled heap, many of its occupants losing their limbs.

CRPF officials said the bus which was the main target of the militants was destroyed fully while another CRPF vehicle was partly damaged. "It is difficult to believe how anyone in that bus could have survived," said a police officer.

Jammu and Kashmir Police chief Dilbag Singh said it could have been a suicide attack. This was confirmed by other officials later.

In a statement to a local news agency GNS, a caller claiming to be a spokesman of JeM said it was a 'fidayeen' (suicide) attack.

All the injured were shifted to the Army's 92 Base Hospital in the Badamibagh cantonment of Srinagar city. Some of them were in critical condition.

Prime Minister Narendra Modi led the nation in condemning the horror.

"The attack ... is despicable. I strongly condemn this dastardly attack. The sacrifices of our brave security personnel shall not go in vain. The entire nation stands shoulder to shoulder with the families of the brave martyrs," he tweeted.

Home Minister Rajnath Singh, who was to visit Bihar on Friday, is set to reach Srinagar instead. Home Secretary Rajiv Gauba cut short his visit to Bhutan to rush to Srinagar for a high-level security meeting on Friday.

"Today's dastardly attack ... is extremely painful and disturbing. I bow to each and every CRPF jawan who has sacrificed his life in service to the nation," Rajnath Singh said.

All traffic on the Srinagar-Jammu Highway came to an abrupt halt. Senior police and CRPF officers rushed to the spot for a post-explosion analysis.

An officer said the CRPF and police would carry out a detailed investigation to understand the circumstances in which the attack took place.

This is the worst single attack in Jammu and Kashmir since a nascent Pakistan-backed separatist campaign began in 1989. It is also the worst attack carried out by militants after the one on the Uri Army camp on September 18, 2016 left 19 soldiers dead -- forcing New Delhi to launch 'surgical strikes' on terrorist camps in Pakistan.

Officials said the reason why so many CRPF personnel were on the move at one go was because the Srinagar-Jammu highway had been shut the past two days due to bad weather. The convoy left Jammu before dawn.