CANBERRA, Dec. 22 (Xinhua) -- Australian Prime Minister Scott Morrison has promised to maintain the war against terrorism in the Middle East after the U.S. announced it was withdrawing from the region.

In a statement released shortly after U.S. Defence Secretary Jim Mattis announced his retirement, Morrison, Defense Minister Christopher Pyne and Foreign Minister Marise Payne warned that the country cannot be complacent about the threat of an Islamic State (IS) resurgence.

Mattis' resignation came in response to reports that President Donald Trump was preparing to pull 7,000 troops out of Afghanistan having already announced a complete withdrawal from Syria where the fight against IS is ongoing.

Morrison, Pyne and Payne on Friday night said that Australia would remain committed to fighting terrorism in the Middle East with the North Atlantic Treaty Organization (NATO) regardless of the U.S. presence in the region.

"Australia will continue to provide security, humanitarian and development assistance in the -region," the trio said in a statement on Friday night.

"Since 2001, the purpose of Australia's mission in Afghanistan has been to support the Afghan government to help contain the threat from international terrorism.

"Both the NATO-led Resolute Support Mission in Afghanistan and the Global Coalition to Counter Daesh (Islamic State) in Iraq and Syria continue to deny terrorist organization safe havens in which to plan and export terror -attacks across the globe, including the Indo-Pacific.

"We cannot be complacent about this threat, including the threat of resurgence by Daesh."

In his resignation letter, Mattis cited differences with President Trump as the reason for his resignation.

"Because you have the right to have a secretary of defence whose views are better aligned with yours on these and other subjects, I -believe it is right for me to step down from my position," he wrote.

Peter Leahy, former chief of the Australian Army, on Saturday said reducing the U.S. presence in Afghanistan would be "a strategic mistake."

"With the U.S. being the catalyst of the coalition, I think what we'll see is many of the other nations get out. I think the consequence for Afghanistan could be dire," he told News Corp Australia.