The United States Sunday started pulling out its nationals from the Shamsi air base in Pakistan at the order of the Pakistani government to leave the base before Dec. 11.

The action followed a NATO air strike at two Pakistani army check posts in Mohmand tribal area near the Afghan border that killed 24 Pakistani soldiers and wounded 13 others on November 26 and dropped U.S.-Pakistani relations to a new low.

An American aircraft arrived in Pakistan to fly out U.S. nationals who boarded the plane amidst strict security, according to Hindustan Times.

Security around the air base has been beefed up in order to ensure the safe evacuation, Xinhua quoted unidentified sources as saying.

The evacuation came eight days after the Pakistani government ordered the U.S. to vacate its air base in the Shamsi area in Pakistan's southwest Balochistan province within 15 days.

Shamsi air base is located some 320 km southwest of Balochistan's capital city Quetta. The U.S. has reportedly been using the air base since 2001 for military operations in Afghanistan and drone strikes in Pakistan's northwest tribal areas bordering Afghanistan.

The U.S. believes the tribal area is a haven for militants who often launch cross-border attacks on the NATO troops in Afghanistan, Xinhua said.

To order the US to vacate the base is one of the three steps taken by the Pakistani government in retaliation for the Nov. 26 incident, which Pakistan described as a blatant violation of its sovereignty.

The other two steps include closure of two border checkpoints for NATO supplies entering Afghanistan from Pakistan and boycott of the Dec. 5 Bonn Conference on Afghan issue.

The UAE foreign minister paid a visit to Pakistan recently to persuade the Pakistani President Asif Ali Zardari to reconsider the decision or to postpone the vacation deadline, but was turned down by Zardari.

Pakistani Prime Minister Yusuf Raza Gilani has rejected requests from Afghan President Karzai, German Chancellor Angela Merkel and the U.S. Secretary of State Hillary Clinton to reconsider the boycott of Bonn Conference.

The three steps so far has not brought an even official apology from the U.S. and NATO apart from condolences and promise of investigation into the incident.

More steps could be taken by the Pakistani side as it has threatened to reconsider the overall arrangement with the U.S., NATO, International Security Assistance Force (ISAF) in the fight against terrorism, Xinhua said.