Militants opened fire on a group of tourists in the Indian-administered region of Kashmir on Tuesday, killing at least two dozen and injuring many more, according to government officials and local media reports.

The attack, in a picturesque district of pine-covered hills and valleys that is popular with Indian travelers, was the worst assault against civilians in the restive region in years, said Omar Abdullah, the chief minister of Jammu and Kashmir, as the region is officially known.

In a social media post, Prime Minister Narendra Modi condemned what he called a “terror attack” and said that “those behind this heinous act will be brought to justice.” Mr. Modi planned to return early from a visit to Saudi Arabia. President Trump said in a post that “the United States stands strong with India against Terrorism.”

No official death toll has been released, and no group immediately claimed responsibility for the attack. India and Pakistan have each laid claim to the region since war broke out following the nations’ partition in 1947, and border skirmishes have long created instability.

The assault occurred just a few miles from the town of Pahalgam in the Baisaran Valley, which is accessible only by foot or on horseback. The 20-minute hike through brush opens on to lush green meadows that are often carpeted by flowers in the summer, making it a big tourist draw.

On Tuesday afternoon, as a group of tourists enjoyed the scenery, gunmen suddenly began firing indiscriminately from behind bushes, said Binu Bhai, 57, who was shot in both his arms and legs and was being treated at a hospital.

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