Russia, Ukraine and the European Union are set to resume gas talks in Brussels on Friday aimed at restoring Russian gas supplies to neighboring Ukraine, the Russian energy minister said Thursday.

The spokeswoman of energy minister Alexander Novak told AFP "we confirm" the meeting is going ahead.

Ukraine's pro-Moscow insurgents showed off their military prowess on Thursday by launching exercises involving heavy tanks that Kiev claims they had covertly received from Russia.

The self-proclaimed "tank biathlon" mirrored maneuvers that Russia has been staging with its former Soviet allies since 2013.

Russia and the United States have reached a "tacit agreement" on ending Syria's bloody crisis, a senior adviser to Syrian President Bashar Assad has said.

"The current U.S. administration wants to find a solution to the crisis in Syria. There is a tacit agreement between the U.S. and Russia to reach this solution," Bouthaina Shaaban said in an interview with state television late Wednesday.

NATO head Jens Stoltenberg told Agence France Presse 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.

"It underlines the importance of Russia making clear what are their intentions," Stoltenberg told Agence France Presse in an interview at NATO HQ in Brussels.

NATO head Jens Stoltenberg told Agence France Presse Thursday there was still a "strong" Russian presence in eastern Ukraine in support of pro-Moscow rebels although a recent ceasefire seems to be holding.

Stoltenberg said he backed the continuation of European Union and United States sanctions against Russia over its involvement in Ukraine until Moscow changes its behavior.

Russia's defense ministry on Thursday said it will hold naval drills in the "east Mediterranean" in September and October, as the West frets over a military buildup by Moscow in Syria.

The exercises include three warships from Russia's Black Sea Fleet, including the Saratov landing ship, the Moskva guided missile cruiser and the Smetlivy destroyer, the ministry said in a statement.

The Syrian military for the first time on Wednesday deployed drones supplied by Russia for its fight against jihadists, a security source in Damascus said.

"For the first time, the army today used drones received from Moscow in operations against extremists in the north and east of the country," the source said, without elaborating on the type of drones or locating exactly where they were deployed.

French President Francois Hollande said Wednesday Egypt had agreed to buy two Mistral warships which France built for Russia before scrapping the sale over the Ukraine crisis.

The deal is the second big military contract this year between France and Egypt, which Hollande said he increasingly views as a strategic partner.

Two new sites north of Syria's Latakia airport could be preparing to receive Russian forces, specialist defense publication IHS Jane's Intelligence Review said on Tuesday citing satellite imagery.

At the Istamo weapons storage complex and the al-Sanobar military complex "preparations appear to be under way to introduce Russian forces," said Robert Munks, editor of the London-based publication.

Russia has increased its deployment of warplanes to Syria but so far they appear to be positioned to defend their own base rather than to mount an offensive campaign, U.S. Secretary of State John Kerry said Tuesday.

"Yes, they have increased aircraft and there are certain kinds of aircraft there which, depending on what Russia's decision is about its long-term intentions, could raise questions," Kerry said.