NATO's secretary-general says the U.S.-led alliance must face "conflict, instability and insecurity" in many different directions, including the "troubling escalation of Russian military activities" in Syria.

Jens Stoltenberg spoke Thursday before a meeting of NATO defense ministers in Brussels.

The ministers will consider the implications of recent Russian military actions in Syria, as well as ongoing measures to retool NATO to meet contemporary security threats.

Stoltenberg said the ministers will assess what must be done to adapt NATO to current and future challenges, including cyberattacks and hybrid warfare.

U.S. Defense Secretary Ash Carter and his counterparts will also discuss the Syria situation.

On Wednesday, Russian warships fired cruise missiles in the first combined air-and-ground assault with Syrian government troops since Moscow began its military campaign in the country last week.