NATO chief Jens Stoltenberg told AFP Thursday he was concerned by Russia's "substantial" military build-up in Syria and urged Moscow to play a constructive role in the fight against Islamic State jihadis.

Russia had its responsibilities in Ukraine too, Stoltenberg said, warning Moscow could expect no let up in Western sanctions unless it helps ensure full implementation of the Minsk ceasefire accords it agreed to in February.

As Moscow announced it would hold naval drills in the eastern Mediterranean later this month, the NATO secretary general said it was essential that Russia make its intentions in Syria completely clear.

The rise in Russian activity "increases the need for them to make sure that they play a constructive role and that they seek to do that in a cooperative and constructive way with the (anti-IS) coalition led by the US," Stoltenberg told AFP in an interview at NATO HQ in Brussels to mark his first year in office.

"I think the first step now is to sit down with the United States and to make clear what are (their) intentions."

In no circumstances should the build-up be used to bolster President Bashar al-Assad -- Russia's long-time Middle East ally -- he said.

After a string of reverses earlier this year, Moscow has begun resupplying Assad with modern military equipment, including aircraft.

- Build-up 'cause for concern' -

Russia also began reinforcing its naval base in the Syrian port city of Tartus and boosting facilities in the Assad's Western stronghold around Latakia.

"It is of course a cause for concern," Stoltenberg said of the military build up.

NATO welcomed recent contacts between Moscow and Washington on Syria, he said, "but at the same time I am concerned because there is also the risk that these forces will be used to support the Assad regime.

"To support the Assad regime is not a constructive contribution to fighting ISIL (nor) to finding a political solution to the conflict," he said.

"The Assad regime is actually part of the problem."

Washington and its allies have up until now insisted that Assad has no future in Syria, but there have been recent signs of a change, perhaps allowing him an interim role until a new government is formed.

US Secretary of State John Kerry said on Saturday that while Assad must ultimately step down, that need not be immediately on reaching a settlement to end a conflict which has now cost some 250,000 lives and created some four million refugees.

Russia has consistently backed Assad as a bulwark against what it sees as a terrorist threat made worse by Western bungling which has allowed IS to seize huge swathes of territory in Syria and neighbouring Iraq.

- 'Strong' Russian presence in Ukraine -

Turning to Ukraine, Stoltenberg said that while the ceasefire now appeared to be holding, there was still a "strong" Russian presence in eastern Ukraine in support of pro-Moscow rebels.

As long as that continued, so should EU and US sanctions against Russia, he said.

"There (are) Russian forces there, there (is) Russian equipment and Russia continues to train and to assist the separatists."

Stoltenberg said it was an "encouraging" sign that the ceasefire accord, brokered by France and Germany, was faring better.

"All the parties of the agreement have still a long way to go before we can say that the agreement is fully implemented but at least it is an encouraging sign that for the first time ... the ceasefire seems to be holding," he said.

"When there is no violence going on, no fighting going on it is easier to make progress also on the other elements of the agreement," which include the withdrawal of heavy weapons and restoration of full Ukrainian control of the border with Russia.

"Russia has to change behaviour if they (are going) to do something with the economic sanctions," he said. "Therefore I think sanctions should continue."

Damaging economic sanctions against Russia come up for review in December, with EU leaders saying a decision to lift them depends on Moscow fully implementing its Minsk commitments.