Amidst a looming potential conflict with Pakistan, India has mandated civil defense exercises

India’s government has ordered officials to carry out civil defense drills in much of the country on Wednesday as it prepares for a potential military conflict with Pakistan, its neighbor and archrival.

Tensions between the nuclear-armed countries, which have fought several wars, have escalated since a terror attack late last month killed 26 people in the India-controlled part of Kashmir, the Himalayan region long-disputed between the two neighbors. India accuses Pakistan of being involved in the terror attack, which Pakistan denies.

India has been making a case for carrying out military strikes on what it calls havens for terrorists in Pakistan, and has threatened a series of punishing acts. Pakistan has promised to respond in kind to any military action by India.

In recent days, exchanges of small-arms fire have broken out along the countries’ border, and a flurry of activity in New Delhi has suggested that an Indian strike could be imminent. Prime Minister Narendra Modi has met repeatedly with his military leaders, and his officials have continued to talk in public about taking action against Pakistan.

Antonio Guterres, the United Nations secretary general, said the tensions between India and Pakistan “are at their highest in years.”

“Targeting civilians is unacceptable, and those responsible must be brought to justice through credible and lawful means,” Mr. Guterres said on Monday. “It is also essential, especially at this critical hour, to avoid a military confrontation that could easily spin out of control.”

In a letter sent on Monday to all of the country’s states and territories, India’s home ministry ordered that drills be carried out to assess the public’s readiness for conflict. In 244 districts — identified as civil defense districts because they are near the border or the coast, or because of other perceived vulnerabilities — all villages will be required to conduct drills, the letter said.

The letter, a copy of which was seen by The New York Times, said the drills should include testing air raid sirens and instructing civilians in how to navigate blackouts and carry out evacuations.

Similar drills have already been conducted near the border in recent days, particularly in Kashmir, where videos have showed schoolchildren being led through duck-and-cover drills and people cleaning bunkers meant to be used during airstrikes.

Nirmal Kumar Singh, 87, a retired senior police officer, said such drills took place during the India-China war in the 1960s, and in the lead-up to the 1971 war between India and Pakistan.

“But I don’t remember such drills after the 1971 India-Pakistan war,” Mr. Singh said.

Much of India’s retaliation in the two weeks since the attack has been nonmilitary, and has been met with countermeasures from Pakistan.

Both sides have expelled the other’s citizens on their soil. Bilateral trade has been frozen, and the airspace shut to each other’s airlines.

The most important measure was India’s pausing its participation in a water treaty that governs the flow of key rivers on which Pakistan’s irrigation system largely depends.

Experts and officials have said that significantly diverting the water flow to Pakistan is a daunting and long-term task that will require India to invest in new dams and infrastructure for storing the water.

But India does have tools at its disposal, such as holding back crucial data sharing or temporarily disrupting the flow of some rivers, making Pakistan’s planning difficult.

“What’s at stake here is not the absolute volume of water, but the predictability and timing of flow,” said Hassaan F. Khan, an assistant professor of environmental studies at Tufts University. “Previously, Pakistan’s water managers could reliably plan around these flows. Now, with sudden and unannounced disruptions, that predictability is eroding. This poses serious operational and planning challenges for a water-stressed country like Pakistan.”

In recent days, there has been evidence that India is making use of some of those tools, as reports have emerged of reduced water flow in the Chenab River. Two Indian officials aware of the situation around the Chenab said India was using a freer hand to clear silt and sediments from dams and then slow the downstream flow of the water to fill the dams up again.

In a time of heightened tensions, any disruption would be seen as punishments kicking in.

One of the officials said such disruption would last a couple of days, until the dams fill again. The local administration, using the megaphones on police vehicles, on Monday warned people venturing onto the riverbed to evacuate to safety, as the water levels would rise soon.

Pakistan’s water-regulating body has expressed concern that disruptions could affect the May-June harvest. It is a busy phase of agricultural activity, particularly in Sindh and Punjab Provinces, where cotton sowing is ongoing and rice cultivation is about to commence.

Khwaja Asif, Pakistan’s defense minister, has warned that any construction by India of infrastructure to divert water in defiance of the treaty would be considered an act of aggression.

“It would amount to a declaration of war against Pakistan,” Mr. Asif said on a TV talk show on Saturday. “Such actions can lead to nations dying of hunger or thirst.”