At least 20 people were killed in Indian-administered Kashmir when gunmen opened fire on tourists on Tuesday, a senior police officer said, with authorities calling it the worst attack on civilians in years.

Prime Minister Narendra Modi decried the "heinous act" in the summer retreat of Pahalgam, pledging the attackers "will be brought to justice."

A tour guide said he reached the scene after hearing gunfire and transported some of the wounded away on horseback.

"I saw a few men lying on the ground looking like they were dead," said Waheed, who gave only one name.

The attack targeted tourists in Pahalgam, which lies about 90 kilometres by road from the key city of Srinagar.

The senior police officer in the region, speaking on condition of anonymity, described a massacre in which at least 20 people had been killed.

No group has claimed responsibility for the attack, but rebels in the Muslim-majority region have waged an insurgency since 1989.

They are seeking independence or a merger with Pakistan – which controls a smaller part of the Kashmir region and, like India, claims it in full.

The killings comes a day after Modi met with US Vice President JD Vance, who is on a four-day tour of India. (AFP)