Indian Prime Minister Narendra Modi said on Tuesday that all those responsible for the car blast in Delhi that killed at least eight people will be brought to justice.

Modi, who was speaking during a visit to Bhutan to open a giant hydropower project, was echoed by his defence minister.

Monday's blast, if confirmed as an attack, would be the first significant security incident since April 22, when 26 mainly Hindu civilians were killed at the tourist site of Pahalgam in Indian-administered Kashmir, triggering clashes with Pakistan.

"The country's leading investigative agencies are conducting a swift and thorough inquiry into the incident -- findings of the investigation will soon be made public," Defence Minister Rajnath Singh said in New Delhi. "I want to firmly assure the nation that those responsible for this tragedy will be brought to justice, and will not be spared under any circumstances."

Crime scene investigators scoured through the wreckage of a car early on Tuesday, hours after the intense explosion.

Senior Delhi police officer Raja Banthia said they were investigating the blast, near the landmark Red Fort in the crowded Old Delhi quarter of the city, under anti-terrorism laws.

However, they have not given details on the cause of the explosion, which also wounded at least 19 people when intense flames ripped through other vehicles near the 17th century structure.

The Press Trust of India news agency reported on Tuesday that the death toll had risen to 12, although that figure has not been confirmed.

Home Minister Amit Shah said late on Monday that security forces were "keeping all angles open", adding that it was "very difficult to say what caused the incident" until forensic samples had been analysed.

Security was increased across New Delhi as both forensic and anti-terrorism agencies searched for evidence. (Reuters/AFP)