"Any use of chemical weapons is unacceptable, cannot go unanswered, and those responsible must be held accountable," Jens Stoltenberg said in a statement.
He said they consider the use of chemical weapons as a threat to international peace and security.
"The Syrian regime bears the full responsibility for this development. NATO has consistently condemned Syria’s continued use of chemical weapons as a clear breach of international norms and agreements," the secretary general added.
The U.S. fired 59 Tomahawk missiles at a Syrian military air base American officials believe was used to carry out a deadly chemical attack Tuesday, the Pentagon confirmed.
The chemical attack in northwestern Syria had killed 100 people and injured around 500 others.
The missiles were fired from two U.S. destroyers in the eastern Mediterranean and hit the air base at approximately 3.45 a.m. Syrian local time Friday (0045GMT). Aircraft, hardened aircraft shelters, petroleum and logistical storage, ammunition supply bunkers, air defense systems, and radars were targeted, the Pentagon said.