BRUSSELS – NATO Secretary General Jens Stoltenberg says that after many years of constant cuts in defense spending, the alliance is now on the right track.

"In 2015 we saw a small increase in defense spending across European allies and Canada," he said. "Our estimates indicate a further increase in defense spending in 2016," he said.

Allies, he added, "must sustain this momentum" and review progress at the Warsaw summit next month.

His comments came after NATO defense ministers agreed on several measures to strengthen the Alliance's defense and deterrence.

"NATO will deploy by rotation four robust multinational battalions to Estonia, Latvia, Lithuania and Poland. And I welcome the commitments made by many allies today to contribute," said Stoltenberg after the ministers' meeting in Brussels on Tuesday.

The former Norwegian prime minister stressed that this sends a "clear message" that if any NATO ally is attacked, the "whole alliance will respond as one."

He said NATO had now fully implemented the "Readiness Action Plan," adopted at the Wales Summit two years ago, and NATO allies were now taking the next steps. Ministers also agreed on tailored measures to enhance defense and deterrence in the Black Sea region with more pre-positioned equipment and supplies.

Ministers also considered the safety, security and effectiveness of NATO's nuclear deterrent; on cyber defense they agreed to recognize cyberspace as an operational domain at the Warsaw summit.

"Treating cyber as an operational domain would enable us to better protect our missions and operations," said Stoltenberg.

The meeting also discussed the freedom of movement of NATO forces across Europe.

"It is vital that our troops and equipment can move without delay. For exercises in peacetime, for reinforcements in an emerging crisis, time is of the essence," said Stoltenberg.

He stressed that NATO allies had cut red tape and updated complex procedures to improve the Alliance's ability to deploy forces at speed.

Stoltenberg argued that NATO's efforts to boost its deterrence and defense depend on the right capabilities and resources.