At a ceremony led by Acting US Defense Secretary Patrick Shanahan, McKenzie pledged to continue the work done by his predecessor, General Joseph Votel, who is retiring.

Votel for his part said that while the Islamic State group had lost its "caliphate" and peace talks with the Taliban appeared to be progressing, the US military presence in Syria and Afghanistan was still necessary.

"It is time to bring these conflicts to a conclusion. This won't be as quick as anyone would like, but it can be done if we continue to keep our objectives and interests in mind, trust our partners, and enable our forces," Votel stressed.

McKenzie, 62, is familiar with Afghanistan after being twice deployed there, and has also served in Iraq.

During his confirmation hearing in early December, he voiced concern about a premature military withdrawal from Afghanistan, where local forces took over at the start of 2015, succeeding NATO troops helping in the fight against the Taliban.