New Delhi: The Jaish-a-Mohammed (JeM) claiming responsibility of Thursday's terrorist attack in Jammu and Kashmir has raised questions over the role of the Pakistan spy agency, ISI, in the gruesome strike, a former CIA analyst and South Asian expert here said.

In one of the deadliest terror attacks in JK's three decades of militancy, a suicide bomber from Pakistan-based JeM terror group rammed an explosives-laden vehicle into a CRPF bus in Pulwama district, killing at least 39 personnel and leaving many critically wounded.

The JeM has claimed responsibility for the attack that took place about 20 km from Srinagar. "The self-proclaimed involvement of JeM in the attack raises serious questions about the role of the ISI in supporting the masterminds of this operation," Bruce Riedel, a former CIA analyst, told PTI.

The terrorist attack, which has direct footprints inside Pakistan, poses the first major challenges to Pakistan Prime Minister Imran Khan, said Riedel, who is now a scholar at the Brookings Institute think-tank.

"This will be a real challenge for Imran Khan, the first serious challenge of his administration," he said. Former Pakistan Ambassador to the US, Husain Haqqani, said Pakistan "would have to act against the JeM".

Anish Goel, who worked in Obama administration's National Security Council, said the "horrific" attack highlights how actively Pakistan-based terrorist groups are operating in Kashmir.

"By claiming such quick credit for the attack, JeM is clearly indicating that they will continue to cause trouble in the region and stoke tensions between Pakistan and India," he said.

"In the wake of this attack, there will likely be increased pressure on Prime Minister Modi to take action against all militant groups still active in Kashmir," Goel told PTI.