NATO chief Jens Stoltenberg said Tuesday the alliance was working on new support measures for Turkey, but insisted the commitment predated Ankara's shooting down of a Russian jet on the Syrian border.

Speaking as foreign ministers from the 28 NATO nations gathered in Brussels, Stoltenberg also reiterated his call for Turkey and Russia to calm the crisis and to find ways to ensure there is no repeat.

"We will work on further measures to assure Turkey's security," Stoltenberg said at the start of the two-day meeting dominated by Syria, Ukraine and relations with a "more assertive" Russia, as well as future plans in Afghanistan.

"I would like to underline... that this is something not related to the incident last week. It has been going on for several years as part of our commitment to an ally," he told journalists at NATO headquarters in Brussels.

The NATO chief did not specify what the new measures would involve but said that the alliance has for many years helped Turkey with its air defences.

Russia said this week that it had deployed its advanced S-400 air defence system at its base in Latakia, Syria, following the shooting down of a Russian fighter jet by Turkish aircraft in November.

In 2012 as the bloody conflict in Syria deepened, NATO allies deployed Patriot anti-missile batteries along Turkey's southern border to prevent any spillover of the fighting.

They have been progressively withdrawn but one battery remains, supplied by Spain although that was due to be pulled out at the end of the year.

Stoltenberg also cited as examples naval deployments by Germany and Denmark in the Mediterranean, or the presence of US aircraft on Turkish soil.

"All of this is relevant for Turkey's reassurance," he said.

The NATO chief also reiterated his call for Turkey and Russia to calm the crisis and to find ways to ensure there is no repeat.

"The focus now should be on how we can de-escalate and calm tensions (and find) mechanisms so that we can avoid the type of incident we saw last week," he said.

The two-day NATO meeting will review measures adopted in the fallout from the Ukraine crisis to upgrade readiness levels and reassure nervous eastern Europe members who were once ruled from Moscow that the alliance will stand by them.

Stoltenberg says the changes, which include creation of a new fast response force, apply globally in what he described as a "dark" security environment.