At least 26 people were killed and 17 others wounded when gunmen opened fire on tourists in Indian-controlled Kashmir, the worst assault in years targeting civilians in a restive region that has seen an anti-India rebellion for more than three decades.

India's Prime Minister Narendra Modi says the gunmen behind a terrorist attack on tourists in the Kashmir region "will not be spared!"

At least 20 people were feared dead after suspected militants opened fire on them in India's Himalayan territory of Jammu and Kashmir on Tuesday, with officials saying it was one of the deadliest such attacks in recent times.