The speculation that a Pakistani domestic airliner entered a no-fly zone in Islamabad before crashing on Wednesday morning, and the failure of the authorities concerned to react for intercepting the plane has raised question marks over the country's air defence system.

When asked to comment as to why the authorities did not intercept the plane despite repeated warnings from the Civil Aviation Authority (CAA) radar, the concerned authorities of the Pakistan Air Force (PAF), Inter-Services Public Relations (ISPR) and the civil defence organisations did not respond to the question directly, The Daily Times reports.

Requesting anonymity, representatives of the organisations said: "It was not the case (sabotage act) as you think. It was the country's plane, it did not come from across the border".

The authorities said that the Airblue plane was not a threat, as it had lost its route in the bad weather.

Meanwhile, Pakistan's defence sources said that the country's air defence had very clear SOPs regarding the security of the Kahuta Nuclear Plant.

"We have clear instructions for intercepting anything having a possibility of reaching the nuclear plant," the source said.

The Airblue plane, which took off at 7.50 a.m. from Karachi to Islamabad, carrying 152 people, including six crewmembers, had crashed behind the mountains of Margalla Hills.

The pilot was reportedly trying to land at Islamabad on schedule at 9.30 a.m., but was told to wait as the airport was busy.

According to eyewitnesses, before it crashed, the plane circled over the F-5 and F-6 sectors of the federal capital, which are close to the Presidency, the PM House, the diplomatic enclave and the Parliament House. (ANI)