His grim assessment comes as the Trump administration considers sending a few thousand more troops to Afghanistan, mainly to boost training and advising of Afghan forces.
After meeting with Chancellor Angela Merkel on Thursday, Secretary-General Jens Stoltenberg expressed his satisfaction with Germany's contributions to North Atlantic Treaty Organisation, including the missions in Afghanistan and Kosovo and the Bundeswehr's deployment in Lithuania to protect the alliance's eastern flank.
"I do not think that we are the first one, who must expand our capacities [in Afghanistan]", Merkel said, adding that she has "no specific plans" about the mission.
Germany's Bundeswehr - the army, navy and air force - has about 180,000 active military personnel, making it the second largest in the European Union, behind France.
"But this is not about returning back to a combat operation in Afghanistan".
Nearly 16 years since Australia first sent forces to Afghanistan after the September 11 attacks, the Turnbull Government is weighing up sending more troops to the conflict.
The NATO-led Resolute Support mission has been operating in Afghanistan since January 2015.
British combat troops left Afghanistan in 2014 but around 500 remain on the ground to train local military forces.
In April, the Taliban announced the beginning of its annual "spring offensive" and last week the armed group stormed and seized a district in the vicinity of Kunduz, in northern Afghanistan. "Those who have been seriously tackling the Afghan issue understand very well what is going on there, so no serious analyst, who is not seeking cheap popularity, will not say that we are supplying weapons to the Taliban", the Russian top diplomat said.
The Afghan government has however welcomed the move but emphasized the need for help regarding the air force.
He added that the goal of the idea of a troop increase is to get the Taliban to the negotiating table.
Below are some numbers that help to contextualize the conflict, which has proved itself to be one of the most costly in US history - not just in terms of dollars, but also in lives. Official sources quoted anonymously by U.S. media said the increase would range from between 3,000 to 5,000 soldiers, including Special Operations forces.
President Donald Trump is expected to receive a proposed new approach to the war this week, according to Theresa Whelan, a Pentagon policy official. "Afghanistan will struggle to curb its dependence on external support until it contains the insurgency or reaches a peace agreement with the Taliban", Coats said.