Indian troops fired on a Pakistani passenger bus in Kashmir on Wednesday, killing 10 civilians in one of the deadliest incidents in weeks of violence in the disputed Himalayan region, stoking tensions between the nuclear-armed rivals.

Pakistan’s Prime Minister Nawaz Sharif called the shelling “naked aggression,” and the government summoned an Indian diplomat to lodge a protest. The Pakistani army contacted its Indian counterpart over a crisis hotline, saying “we reserve the right to respond at the place and time of our choosing.”

READ MORE: Kashmir border erupts in gunfire between India, Pakistan

The Pakistani military said it had already fired back at Indian positions, killing seven soldiers, but there was no word on casualties from the Indian side.

The Indian military said in a statement that Lt. Gen. Ranbir Shah had spoken to his Pakistani counterpart and expressed “grief” over the civilian casualties. But he insisted that India’s “retaliatory” fire had only been directed at locations where Pakistan had fired first.

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Kashmir is split between Indian and Pakistani areas of control and claimed in its entirety by both countries. The South Asian rivals fought two of their three wars over Kashmir, and it remains one of the world’s most intractable conflicts.

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Waheed Khan, a Pakistani police official in the region, said the Indian shelling also hit villages near the Line of Control separating the two sides, killing another two civilians and causing others to flee. At least three Pakistani soldiers were also killed, the military said, adding that Indian troops had fired on an ambulance.

The latest exchange of fire came a day after the mutilated body of an Indian soldier was found in Kashmir. The Indian military did not say whether the soldier was killed by Pakistani soldiers or Kashmiri rebels, who have been fighting against Indian rule since 1989.

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“Retribution will be heavy for this cowardly act,” Col. Rajesh Kalia, an Indian army spokesman, said after Tuesday’s attack.

Tensions have soared in Kashmir since militants attacked an Indian army base there in September, killing at least 17 soldiers. India said the militants were supported by Pakistan, charges denied by Islamabad. The two sides have traded blame for the cross-border fire and have accused each other of violating a 2003 cease-fire in Kashmir.

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Last week, the Pakistani army claimed to have shot down a small Indian drone in Kashmir, a day after the navy said it had intercepted Indian submarines entering the country’s territorial waters in the Arabian Sea.