Pakistan's media has reported that the country's military is on high alert over a possible attack by India.
The reported move on Monday came as New Delhi said it had handed over to Islamabad a letter from the only surviving attacker from last month's rampage in Mumbai, which left more than 170 people dead.
The letter said that all 10 attackers were from Pakistan, India's foreign ministry said.
Kamal Hyder, Al Jazeera's correspondent in Pakistan, said Pakistan's media had attributed its reports to military sources, who were confirming that the navy, air force and army were on red alert.
"The Pakistani air force have been seen visibly in a number of locations flying close to the Pakistani-India border in what is being described as an aggressive patrolling mode, following reports that India is planning pre-emptive strikes against locations in Pakistan," Hyder reported.
Hyder said that the chiefs of Pakistan's three armed forces were holding what had been described as an emergency meeting at general headquarters in Rawalpindi.
'Exploring all options'
New Delhi has said it will "explore all options" to push Islamabad to crack down on cross-border attacks.
It also called for more pressure to be put on Pakistan to co-operate with the investigation into the attacks, which it says were carried out by the Pakistan-based Lashkar-e-Taiba.
"While we continue to persuade the international community and Pakistan, we are also clear that ultimately it is we who have to deal with this problem," Pranab Mukherjee, India's foreign minister, said.
Tensions between the two nations following the Mumbai attacks have strained a peace process begun in 2004. They have fought three wars since India's partition and independence from Britain in 1947.
The office of Yousef Raza Gilani, Pakistan's prime minister, quoted him as telling Pakistan's high commissioner to India: "... if war is imposed upon us the whole nation would be united and the armed forces are fully capable of safeguarding and defending the territorial integrity".
The Reuters news agency quoted a Pakistan airline official as saying the Pakistani air force had conducted an exercise on Monday causing delays to two civilian flights.
"I think the media is building up a scenario in which one may get the impression that we are close to war"
Asad Durrani, former head of Pakistani intelligence
"Two of our flights were delayed for some time because the PAF was conducting some exercises, but now everything is back on normal," Muhammad Latif, a spokesman for Pakistan International Airlines, said.
The flights were delayed at the airport in Lahore, near the Indian border, Latif said, while dismissing television reports of a high alert at Pakistani airports.
An air force spokesman declined to comment when asked about an exercise, saying only: "In view of the current environment, the PAF has enhanced its vigilance."
Divya Gopalan, Al Jazeera's correspondent in India, said that the Indian media was looking at Monday's reports with some degree of scepticism.
"They are saying that Pakistan is creating an artificial war hysteria to divert attention from the fact that they are under pressure from India to deal with the Mumbai terror attacks."
Asad Durrani, the former head of Pakistani intelligence, told Al Jazeera that Pakistan was asked to do more to deal with individuals behind the Mumbai attacks. He dismissed the notion that the two states were on the brink of war.
"The media is building up a scenario in which one may get the impression that we are close to war.
"This is not the stage that the two forces are going to go on that sort of alert."
But Brigadier-General Naeem Salik, a retired Pakistani military analyst, told Al Jazeera from Islamabad: "There have been very threatening statements [from India] saying that they do not rule out military options, and they have been talking about punishing Pakistan.
"So it is obviously natural for Pakistan to heighten its alert levels and be on guard. We cannot let the Indians have a free-run and it is a response to what is happening across the borders."
Ravi Sawhney, an Indian security analyst, told Al Jazeera: "It is not threatening talk at all. It is talking facts. We have been assaulted. A terror attack was launched on us. And the perpetrators of that attack were Pakistanis.
Last week, Pakistan summoned a senior Indian diplomat in Islamabad to protest against recent alleged airspace violations by Indian warplanes.
Pakistan said its own fighter jets were scrambled to chase off the intruders, but it also played down the incident by describing the violations as "technical" and "inadvertent".
India denied any violation of Pakistani airspace.