New Delhi: US aviation regulator FAA on Thursday warned America's airlines and their pilots that there is risk involved in operating flights in Pakistan airspace due to "extremist or militant activity", according to an official document.

"Exercise caution during flight operations. There is a risk to US civil aviation operating in the territory and airspace of Pakistan due to extremist/militant activity," said the US Federal Aviation Administration (FAA) in a notice to airmen (NOTAM) dated December 30, 2019.

The NOTAM is applicable to all US-based airlines and US-based pilots.

The US regulator said in its NOTAM that there continues to be a risk to US civil aviation sector from attacks against airports and aircraft in Pakistan, particularly for aircraft on the ground and aircraft operating at low altitudes, including during the arrival and departure phases of flights.

"The ongoing presence of extremist/militant elements operating in Pakistan poses a continued risk to US civil aviation from small-arms fire, complex attacks against airports, indirect weapons fire, and anti-aircraft fire, any of which could occur with little or no warning," it said.

The FAA said that while, to date, there have been no reports of man-portable air defence systems or MANPADS being used against the civil aviation sector in Pakistan, some extremist or militant groups operating there are suspected of having access to these MANPADS.

"As a result, there is potential risk for extremists/terrorists to target civil aviation in Pakistan with MANPADS," it said.

The regulator added that pilots or airlines must report safety or security incidents - which may happen in Pakistan - to the FAA.

Pakistan on July 16 last year opened its airspace for India after about five months of restrictions imposed in the wake of a standoff with New Delhi.

Following the Balakot airstrikes by the Indian Air Force, Pakistan had closed its airspace on February 26 last year.

Pakistan in October last year had denied India's request to allow Prime Minister Narendra Modi's VVIP flight to use its airspace for his visit to Saudi Arabia over the Kashmir issue.