The remarks came after Afghan President Hamid Karzai announced at a ceremony in Kabul that his country's security forces are taking over the lead for security nationwide from the NATO coalition.

"This decision marks an important milestone. As Afghan forces step forward, ISAF's role will shift from combat to support. We will no longer plan, execute or lead combat operations," Rasmussen said in a statement.

Around 97,000 troops from NATO countries and its partners are stationed in Afghantain. NATO has decided to withdraw all combat troops from the Central Asia country by the end of 2014 and leave a much smaller force behind for a new mission.

Called "Resolute Support," Rasmussen said the goal of the new mission "is to train, advise and assist the Afghan forces."

"NATO and Afghanistan, along with our partners, will continue this journey together, based on a new relationship that will remain strong for the years to come," he said.