President Donald Trump vowed that the U.S. stands strong with India against "terrorism" after gunmen reportedly killed 20 people in the Kashmir region.

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, in a picturesque district of India, was the worst assault against civilians in the restive region in years, its leader said.