BRUSSELS / ANKARA 

Turkey has been a significant ally in international efforts to counter terrorism, the chief of the NATO alliance said on Wednesday.

"Turkey has been a very important ally in the fight against terrorism," Secretary-General Jens Stoltenberg told reporters.

His remarks followed a ceremony at NATO headquarters in Brussels commemorating the Sept. 11, 2001 terrorist attacks.

Stoltenberg said he had spoken with Turkish Foreign Minister Mevlut Cavusoglu on the phone on Wednesday, adding that Turkey is "one of the lead" nations in Afghanistan.

He added that Turkey is also a key country in the fight against Daesh in Iraq and Syria, and without its contributions it would be "much harder" to make gains against Daesh.

The NATO chief also praised cooperation between Turkey and the U.S. in northern Syria.

Remembering 9/11

"This memorial symbolizes that while 9/11 was an attack against the United States, it was also an attack against all NATO allies," Stoltenberg said, speaking in front of a wreckage from the Twin Towers.

He added that in the wake of the attack, for the first and only time in NATO’s history, the alliance invoked its collective defense clause – Article 5, which says an attack against any country in the alliance is an attack against all.

Slain al-Qaeda leader Osama Bin Laden claimed responsibility for the Sept. 11, 2001 attacks on New York's Twin Towers as well as on the Pentagon.

Nearly 3,000 people were killed, including citizens of 77 countries, on that day 18 years ago.

Hundreds more first responders have died in the nearly two decades since from illnesses they developed while trying to save lives in the smoky, dust-filled ruins of the attacks, particularly in New York City, where thick plumes of dark ash filled the air.

"On 9/11, terrorists destroyed the Twin Towers. But they will never destroy our way of life; our free and open societies," said Stoltenberg.