Since the horrific terrorist attack in Kashmir last week, the Indian prime minister, Narendra Modi, has spoken on the phone with more than a dozen world leaders. Diplomats from 100 missions in India’s capital have filed into the foreign ministry for briefings, officials said.

But the effort is largely not about rallying help to de-escalate India’s dangerous face-off with Pakistan, which it accuses of having “linkages” to the attack. Instead, according to four diplomatic officials aware of the discussions, New Delhi appears to be building a case for military action against its neighbor and archenemy. Without naming Pakistan, Mr. Modi in a speech on Thursday promised severe punishment and the razing of terror safe havens.

In an indication of how volatile the situation remains, the security forces of the two sides have exchanged intermittent small-arms fire across the border, Indian officials said on Sunday. One official said the exchange of fire had happened two of the past three nights, while a second official said it was three consecutive nights.

In Kashmir, Indian forces have also begun a sweeping clampdown, arresting hundreds, as they continue their hunt for the perpetrators.

Earlier, India declared its intention to disrupt the flow of water to Pakistan, whose irrigation system depends largely on upstream rivers. It also ordered the immediate departure of some staff members at Pakistan’s diplomatic mission as well as of Pakistani citizens visiting India.

Pakistan, for its part, has said it will suspend participation in bilateral treaties, including one that affects the “line of control” demarcating the frontier between the two countries in the disputed areas where a cease-fire had held for several years.