SRINAGAR, India — India blamed Pakistan on Wednesday for a militant attack that killed 26 people in Indian-held Kashmir, downgrading diplomatic ties and suspending a crucial water-sharing treaty that has withstood two wars between the nuclear-armed rivals.

India announced a series of retaliatory actions, and hinted at the possibility of more, in response to what it called Pakistan’s support of terrorism on Indian soil.

India announced a raft of measures to downgrade its ties with Pakistan on Wednesday, a day after suspected militants killed 26 men at a tourist destination in Kashmir in the worst attack on civilians in the country in nearly two decades.