A Pakistani Kashmiri takes part in a protest in Islamabad on Sept. 22. It was being led by the All Parties Hurriyat Conference, which is expressing solidarity with Kashmiris who are resisting Indian rule. (B.K. Bangash/AP)

Military officials are calling it a routine exercise, but the thunderous spectacle of Pakistani fighter jets touching down on a major highway Wednesday and Thursday, with commercial flights suspended and traffic blocked for hours, has fueled public speculation that something much more ominous is afoot.

The display of military readiness, which included a late-night jet flyover Thursday above this capital city, has come amid an unusually tense showdown with India, Pakistan’s nuclear-armed rival, following a militant attack Sunday that killed 18 Indian soldiers in the disputed border region of Kashmir. The air exercise led to the closure of commercial airspace over several regions of the country and triggered a sudden drop in the nation’s stock market. 

Indian officials have accused Pakistan of sending the armed attackers across the de facto border into the Indian portion of Kashmir. Prime Minister Narendra Modi, under public pressure to retaliate for Sunday’s assault, vowed that those behind the “despicable attack” would not go unpunished. So far, though, his government has taken no action. 

Pakistani officials have strongly denied the charges, and its military leaders have declared that they are prepared to defend Pakistani territory from any attack by India, and also to launch a “counter-offensive” in case of an Indian strike. The two neighboring countries have been adversaries for decades and have fought four wars. 

On Thursday, Indian naval officials issued a high alert for coastal areas after school children claimed to have seen four men moving “suspiciously” near a naval facility near the city of Mumbai, according to the Press Trust of India. Schools and some public buildings in the area were shut while a manhunt was conducted, and security was tightened at other coastal facilities.

In New York, meanwhile, Pakistan and Indian officials have carried on a parallel war of words at the U.N. General Assembly. Pakistani Prime Minister Nawaz Sharif delivered a strident speech Wednesday, denouncing what he called ongoing Indian repression against unarmed protesters in Kashmir.

In turn, Indian officials at the meeting criticized Sharif for praising a young militant commander in Kashmir, Burhan Wani, who was killed in July by Indian forces there, sparking days of rioting. Wani was a hero to many Kashmiri Muslims but had called for violent attacks on Indian forces. One Indian official at the U.N. said it was “shocking” that Sharif could “glorify a self-declared. . . terrorist.”

The war of words gave an air of patriotic saber-rattling to domestic media coverage of Pakistan’s military exercises. On Thursday, one TV news channel broadcast repeated images of Pakistani Mirage and F-16 fighter jets touching down and taking off along the 240-mile high-speed motorway between capital and the city of Lahore.

“In response to Indian threats, Pakistani forces have also accelerated their preparations to safeguard the motherland and give a befitting response to any attack by the enemy,” the accompanying voice-over repeated each time. It said the closure of air space because of these maneuvers and to precautions in northern Pakistan resulted in 10 domestic flights being cancelled. 

Some Pakistani press outlets also reported that the military exercises had caused a sharp drop in the nation’s stock market, based in Karachi. 

Pakistani air force officials described the activities as part of a routine air defense exercise, code-named “High Mark,” which they said had also been conducted in 2010. One official said that Pakistan’s new wide highways have been built in part so planes can land there in emergencies. “It doesn’t mean we are going to war. We are just exercising to check and increase our capacity,” he said.

But two security officials, speaking on condition of anonymity to discuss a sensitive issue, said the exercises were more than routine and are part of a strategy to foil any possible Indian attack. “We don’t believe in aggression, but we will take any required action in our defense,” said one official. “We have taken air and ground defense measures. We don’t want a war but we have the right to defend our land.”

Several Pakistani analysts said official concern about Indian attacks was justified, given the high degree of tension and the especially deadly nature of Sunday’s attack. Indian press reports have speculated about possible strikes against what India calls “terrorist infrastructure targets,” such as reported militant training camps inside Pakistan. 

Shahid Latif, a former deputy air force chief, said it was important to remind India of Pakistan’s military and nuclear strength as a deterrent to any impulsive act. He said the air force now has upgraded F-16s and JF-17 Thunder fighter planes.

“India is very frustrated and it could do something rash, such as launching surgical strikes,” he said. “Our forces are well prepared to counter any Indian attack, our air force is doing the exercises and the motorway is also being used for that.” If India attacks, he said, “our military command knows what it has to do.”

Pakistani officials said that on Wednesday and Thursday, Pakistani fighter planes landed and took off repeatedly at several points along the six-lane highway linking Islamabad with the eastern city of Lahore, near the Indian border. Highway officials said they were informed only shortly before each landing, and that they then diverted traffic to other roads.

They said the fighter jets also landed Wednesday on another six-lane motorway that connects Islamabad to the western city of Peshawar to Islamabad. They said flights from Islamabad to the northern areas of Gilgit, Chitral and Skardu were suspended and will remain so for the next several days, with local airports being used by the air force. 

Annie Gowen in New Delhi contributed to this report.