Syrian government troops backed by Russian air strikes advanced against rebels Sunday, as a rights group accused Moscow of being behind a new type of cluster bomb in the conflict.

Twelve days into Russia's aerial campaign, the troops were gaining ground on two fronts: in central Hama province around the Damascus-Aleppo highway, and in the northern part of regime stronghold Latakia province.

Iran on Friday declined to confirm a claim by a U.S. official that Russian missiles targeting rebels in Syria crashed in its territory this week.

"We don't confirm" this information, foreign ministry spokeswoman Afkham Marzieh said when asked about the claim.

Russian President Vladimir Putin's bullish entry into the Syrian conflict has worked wonders for his popularity in neighboring Iraq, where some await "Hajji Putin" like a savior.

Sitting at his easel in his central Baghdad workshop, painter Mohammed Karim Nihaya touches up a portrait of Putin he copied from the Internet.

Regime forces advanced Thursday in western Syria in a "vast offensive" against armed opposition groups, as NATO voiced alarm at escalating Russian military activity in the war-torn country.

Moscow has dramatically stepped up its nine-day-old air war against foes of President Bashar Assad, with heavy bombing by warplanes and cruise missile strikes from the Caspian Sea.

Russia dramatically escalated its air war in Syria Wednesday, unleashing heavy bombardments and cruise missile strikes from the Caspian Sea as cover for a major Syrian army ground offensive against rebels.

Russian President Vladimir Putin said Moscow had begun synchronizing its strikes with the army's ground movements as the Kremlin voiced willingness to make contact with Western-backed rebels that Washington and its allies accuse it of targeting. 

French President Francois Hollande did not suggest forming an alliance between Syrian President Bashar Assad's forces and the moderate opposition Free Syrian Army, an aide said Wednesday, contradicting comments by Russian leader Vladimir Putin.

"The president spoke of the necessary presence of the Syrian opposition around a future negotiating table. The rest is not a French idea," a member of Hollande's entourage told reporters during a visit to Strasbourg.

Russian airstrikes in northern Syria have destroyed the arms depots of a U.S.-backed rebel faction and wounded several of its fighters, the group told AFP on Wednesday.

"At 5:30 pm (1430 GMT) on Tuesday, our bases in the western parts of Aleppo province were targeted by Russian airplanes... which completely destroyed our warehouses," said Mustafa Halabi, a spokesman from the Suqur al-Jabal group.

The Russian air campaign in Syria has thrown an immense obstacle in the way of President Recep Tayyip Erdogan's vision for the future of the country, enraging the Turkish leader and leaving Ankara increasingly sidelined.

Russia's air strikes are aimed at propping up the regime of President Bashar Assad, the very ruler who Turkey has for years insisted must be ousted if there is to be any solution to the Syrian crisis.

President Petro Poroshenko said on Wednesday a "real truce" had begun in eastern Ukraine but a long-lasting peace with pro-Kremlin insurgents would still take some time.

His upbeat comments come with EU nations -- struggling with a migrants crisis -- hoping for signs of a de-escalation after the conflict stoked tensions in eastern Europe.

Turkish Prime Minister Ahmet Davutoglu Wednesday said only two Russian air strikes in Syria had targeted Islamic State jihadists and warned that Ankara would not make any concessions about the security of its borders.

His comments came after Russian warplanes twice violated the airspace of key NATO member Turkey in the last days.