Syrian regime forces backed by Russian air strikes launched an offensive in the central province of Homs on Thursday, Syria's state media and a monitor said.

State television, citing a military source, said the army had begun a military operation in north and northwest Homs province "with the goal of restoring security and stability to the villages and towns in the area".

Russia and the United States said Wednesday that they were close to agreeing a deal to avoid clashes between their warplanes over Syria, as regime forces bombarded rebels near Damascus.

Fighting was also reported on the ground in the northern city of Aleppo, where jihadists from the Islamic State group were advancing against rebels.

Turkish Prime Minister Ahmet Davutoglu on Wednesday warned the United States and Russia against "unacceptable" military and political support for Syrian Kurdish forces fighting the Islamic State (IS) group in Syria.

Turkey earlier summoned U.S. and Russian envoys to warn against supplying arms and support for Syrian Kurdish forces fighting the Islamic State (IS) group in Syria.

Syrian regime bombing raids struck rebel positions around Damascus on Wednesday in a bid to dislodge insurgents entrenched on the outskirts of the capital, a military source told AFP.

"The army began a military operation this morning with the aim of expanding a security zone around areas controlled" by the government, the source said.

Hong Kong's flagship carrier Cathay Pacific has suspended flights over Iran and the Caspian Sea after the European Aviation Safety Agency (EASA) issued a warning about missiles being fired at Syria.

The air safety fears come as investigators issued their final report into the fate of Malaysia Airlines flight MH17, concluding it was shot down by a Russian-made BUK missile fired from war-torn eastern Ukraine.

The Islamic State group called on Muslims Tuesday to wage jihad on Russia and the United States, as Moscow announced it had intensified its air campaign against Islamic militants in Syria.

The call came as Russian President Vladimir Putin criticized Washington for refusing to cooperate with Moscow in its Syrian campaign, which is having an increasingly dramatic effect.

President Vladimir Putin on Tuesday slammed Washington for refusing to share intelligence with Russia on Syria, accusing it of muddled thinking.

"I believe some of our partners simply have mush for brains," Putin said, expressing some of his strongest criticism yet of Washington's handling of the Syrian crisis.

The head of Al-Qaida's Syrian affiliate has called on jihadists in the Caucasus to attack Russian civilians and soldiers in retaliation for Moscow's air strikes in Syria.

"If the Russian army kills the people of Syria, then kill their people. And if they kill our soldiers, then kill their soldiers. An eye for an eye," Abu Mohamed al-Jolani, the head of Al-Nusra Front, said in an audio recording released late Monday.

Russian Foreign Minister Sergei Lavrov on Tuesday described the shelling of Moscow's embassy in Damascus as an "act of terror" aimed at intimidating those who support Moscow's bombing campaign in Syria. 

"It is a clear act of terror meant to scare supporters of fighting terrorism," Lavrov told reporters in Moscow, after two rockets struck the Russian embassy compound.

Israeli artillery targeted Syrian army posts Tuesday after two rockets fired from Syria hit the Israeli-occupied sector of the Golan Heights, Israel's army said.

"In response to the rocket fire, IDF artillery targeted two military posts of the Syrian Armed Forces in the central Syrian Golan Heights," an army statement said.