The United States has warned its air carriers to avoid Pakistani airspace citing the probable threat of attacks on its airlines. In a statement, the US Federal Aviation Administration (FAA) said that the commercial and the state carriers could be a target of Pakistani extremist and militant groups.

The commercial and state carriers are "advised to exercise caution when flying into, out of, within, or over the territory and airspace of Pakistan due to extremist or militant activities" in the country. 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 defense 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 a potential risk for extremists/militants 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.

The development has come two days after several thousand protesters attacked the United States embassy in Baghdad in retaliation to the US airstrikes that killed more than two dozen pro-Iran fighters.

A stream of men in military fatigues, as well as some women, marched through checkpoints that usually restrict access to Baghdad's high-security Green Zone.

They waved flags in support of the Hashed al-Shaabi, a network of armed groups which has been largely incorporated into the security forces.

On Sunday, at least 25 fighters from a hardline Hashed faction known as Kataeb Hezbollah (Hezbollah Brigades) were killed in US airstrikes on a base in western Iraq. The strikes were in response to escalating rocket attacks on Iraqi bases where the United States' forces are deployed.

After the protesters stormed the US embassy, the US security forces inside the compound of the building fired tear gas and flash bangs to disperse the crowds, which had ignored calls over megaphones to back away from the embassy.

(With inputs from PTI)