At least 20 people were feared killed after suspected militants opened fire on tourists in India's Jammu and Kashmir territory on Tuesday, three security sources said, the worst attack on civilians in the troubled Himalayan region for years.

The attack appeared to be a major shift in the regional conflict in which tourists have largely been spared from violence.

Suspected militants opened fire in a popular tourist valley in the Indian-administered Kashmir, in one of the worst attacks on civilians in recent years, according to government officials.