Indian Prime Minister Narendra Modi on Tuesday called a deadly car explosion in the heart of the capital that killed at least eight people a "conspiracy", vowing those responsible will face justice.

Police are yet to give exact details of what caused Monday's blast near the historic Red Fort, one of India's most well-known landmarks, and the site of the annual prime minister's Independence Day speech.

It is 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.

"I assure everyone that the agencies will get to the bottom of the entire conspiracy," Modi said, in a speech during a state visit to neighbouring Bhutan, without giving further details.

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