BRUSSELS -- NATO's chief on Thursday signalled its readiness to deploy forces if needed to protect Turkey against any threat from Russia, as the alliance agreed on more changes to meet today's security threats.

"All of this sends a clear message to all NATO citizens. NATO will defend you, NATO is on the ground, NATO is ready," alliance secretary-general Jens Stoltenberg said.

The actions came at a defence ministers' meeting that was overshadowed by growing concerns over Russia's recent military actions in Syria.

The new measures agreed upon include finalized plans, including command and control arrangements, for a NATO Response Force of up to 40,000, twice the current size, and the creation of new NATO headquarters offices in Hungary and Slovakia, Stoltenberg announced at a news conference.

"We see an escalation of Russian military activity in Syria," Stoltenberg said. "And the ministers agreed that Russia's military escalation in Syria raises serious concerns."

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 Middle Eastern country last week.

Over the weekend, Turkey, a NATO member, reported back-to-back violations of its airspace by Russian warplanes.

Stoltenberg, NATO's top civilian official, called Russia's actions and unwavering support for beleaguered Syrian President Bashar al-Assad "not helpful."

In remarks to reporters earlier, Stoltenberg said the alliance was ready to send help to Turkey if required.

"NATO is able and ready to defend all allies, including Turkey, against any threat," he said.

He said NATO had already increased "our capacity, our ability, our preparedness to deploy forces, including to the south, including in Turkey, if needed."

Pressed later at the news conference about what NATO intended to do to assist Turkey, which shares a border with Syria, Stoltenberg indicated that the mere existence of the beefed-up response force, as well as a newly created and highly nimble brigade-sized unit able to deploy within 48 hours, may suffice.

"We don't have to deploy the NATO Response Force or the spearhead force to deliver deterrence," Stoltenberg said. "The important thing is that any adversary of NATO will know that we are able to deploy."

Russia called its penetration of Turkish airspace a minor incident that was unintentional, but NATO issued a toughly worded statement Monday insisting such violations cease.

"We are constantly assessing the situation also with the Turkish government," Stoltenberg said Thursday, adding that he would be meeting later in the day with Turkish Defence Minister Mehmet Vecdi Gonul.

Earlier, British Defence Secretary Michael Fallon accused Russia of acting chiefly in Syria not to attack the Islamic State terrorist organization but to support Assad, thus making a serious situation "much more dangerous." NATO officials have expressed fears there could be an encounter, accidental or otherwise, between Russian planes and air forces of the U.S.-led coalition attacking Islamic State in Syria.

German Defence Minister Ursula von der Leyen said Russia must recognize that if it targets opposition groups in Syria that are fighting Islamic State, "Russia will strengthen IS and this can be neither in the Russian interest, nor in our interest."

U.S. Defence Secretary Ash Carter and his counterparts from the 27 other NATO nations had already been scheduled to meet in Brussels. The Thursday meeting, their first since June, was intended to implement changes in the alliance ordered by President Obama and other NATO leaders at 2014 Wales summit, a process that is expected to last until the next summit in 2016 in Warsaw.

"We are facing many challenges from many different directions," Stoltenberg said. "Conflict, instability and insecurity."

"We will assess what we have to do to adapt NATO to current and future challenges," he said--including cyberattacks and the blend of conventional and unconventional tactics commonly known as hybrid warfare.