NEW DELHI — The only gunman captured after last month's deadly attacks in Mumbai insisted in a letter handed to Pakistan Monday that he is a Pakistani citizen, the latest effort by Delhi to get Islamabad to crack down on the masterminds allegedly behind the rampage.

Islamabad has not acknowledged that Kasab is Pakistani and has said it is waiting for proof of his citizenship before it will take further action.

India's Foreign Minister Pranab Mukherjee said that if Pakistan doesn't deal with those responsible, India was prepared to "take all measures necessary as we deem fit to deal with the situation."

"Ultimately, it is we who have to deal with this problem," Mukherjee told a meeting of Indian envoys in Delhi, without elaborating.

Suspect Mohammed Ajmal Kasab wrote that he and the nine other gunmen involved in the Nov. 26 attack all came from Pakistan, India's Foreign Ministry said. He also requested a meeting with Pakistani envoys, the ministry said.

The investigation into the attacks that killed 164 people has heightened tensions between India and Pakistan, longtime rivals that have fought three wars since gaining independence from Britain in 1947.

On Monday, fighter jets flew low near several of Pakistan's major cities in exercises that briefly delayed two Pakistan International Airlines flights, airline officials said.

Pakistan's air force declined to comment on the flights but issued a statement saying that "in view of the current environment, the Pakistan Air Force has enhanced its vigilance."

Now that it has Kasab's letter in hand, Pakistan faces pressure to answer to India's demands that it turn over wanted leaders of the Pakistan-based militant group, Lashkar-e-Taiba, accused of plotting the attacks.

"We are examining the letter," Pakistan High Commission spokesman Abid Saeed said late Monday.

Pakistan has moved against both Lashkar-e-Taiba and Jamaat-ud-Dawam, a charity India and the international community say is a front for Lashkar, but has refused to take further action until it has proof of Pakistani involvement. Islamabad has not acknowledged that Kasab is Pakistani, saying it has had no confirmation of his citizenship.

Kasab is the only accused gunman to survive the three-day standoff with police that followed the attack on two luxury hotels, a train station, a Jewish center and other sites in India's bustling commercial capital.

Calling Lashkar-e-Taiba an international threat, India's foreign minister again urged tough action.

"This terrorist infrastructure in Pakistan is the greatest terrorist danger to peace and security of the entire civilized world," Mukherjee told the meeting of Indian diplomats.

"We have so far acted with utmost restraint and are hopeful that (the) international community will use its influence to urge (the) Pakistani government to take effective action," he said.

Pakistan's foreign minister has said Islamabad is prepared to cooperate, but must heed its own laws.

"As far as handing over of people (to India), we have our own laws and we have to operate within the ambit of our own laws," he said Sunday in Multan, Pakistan.

He urged restraint, saying: "We should act sensibly, not emotionally. We do not want war. We want peace."