Rasmussen took the helm at NATO in August 2009 and has been chief during a turbulent period in Afghanistan, where NATO-led forces have been engaged in fierce fighting against Taliban insurgents.

More than 2,000 U.S. military and civilian personnel have been killed in 11 years of war in Afghanistan as well as more than 400 British soldiers and 158 Canadians.

The NATO mission has also seen a surge in attacks on foreign troops by gunmen wearing Afghan police or army uniforms.

At least 52 members of the NATO-led International Security Assistance Force have been killed this year in rogue attacks, leading to a temporary suspension of some joint operations with Afghan forces last month that has now been largely lifted.

Last year, a NATO bombing campaign in Libya, approved by the U.N. Security Council, helped underpin an Arab Spring uprising that ended Muammar Gaddafi’s rule.

NATO chiefs are traditionally appointed for a four-year term with the possibility of extending by a year. There have been cases where a NATO chief’s term has been extended for a few months beyond that but in recent times, secretaries-general have not been given full second terms.

(Additional reporting by James Mackenzie, Paul Taylor and Matt Spetalnick; Editing by Alison Williams)