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.

Relations between India and Pakistan are cratering following a deadly attack in the disputed Himalayan region of Jammu and Kashmir that left more than two dozen tourists dead.

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.