Washington had earlier warned Syrian President Bashar al-Assad and his Russian allies that violations of a 'de-escalation' zone agreed by the United States and Russia last year would have 'serious repercussions'.
Since fighting broke out in his country in 2011, Assad has travelled only rarely, but has made three publicly-acknowledged visits to Russia, each time for meetings with Putin.
President Donald Trump warned Russia on Wednesday that missiles ‘will be coming’ in response to the 7 April gas attack.
President Bashar al-Assad’s government has vowed to retake all of Syria from rebels who have lost large swaths of the territory they have held in a war now entering its eighth year.
Russia, which has a long-term military garrison in Syria, has said it wants foreign forces to quit the country eventually.
The Syrian government led by Assad denies reports by the commission documenting widespread war crimes committed by government-backed forces and Syria's security services.
The troops being sent as reinforcements are considered to be the Syrian army’s elite division, which has the best training and equipment.
White House spokesperson Sean Spicer triggered an uproar on Tuesday by saying Adolf Hitler did not use chemical weapons.
US Secretary of State Rex Tillerson, meanwhile, blamed Russian inaction for helping fuel the chemical weapons attack it had reacted to.
Tehran is Syrian President Bashar al-Assad’s main regional ally and has provided military and economic support for his fight against rebel groups.
Western governments have unanimously pointed the finger at Syrian government forces for the attack.
Syrian President Bashar al-Assad said his family did not "own" the country they have run for 46 years.
Mevlut Cavusoglu has describing the situation in Aleppo as critical, adding President al-Assad was unfit to rule.
The Syrian military announced the major new offensive against the area on Thursday night.
A 48-hour ceasefire in Aleppo announced by Russia on Thursday has had little impact on fighting.
Mohammed Alloush said the peace talks had also failed to secure the release of thousands of detainees.
There has been no indication of when a full assault on Raqqa city might take place.
The city of Aleppo, about 50 km south of the Turkish border, is divided between the government & rebels.
