MUZAFFARABAD, Pakistan — 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.”

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.