President Assad's survival is critical to maintaining Russia’s economic and military interests in Syria.
Turkish troops backed a rebel offensive to drive IS militants out of one of the last areas of the Syrian side of their border.
Russia's only naval base in the Mediterranean is located in Syria, at Tartus, and they are determined to keep it.
Saudi Arabia is a major provider of military and financial help to the rebels, including those with Islamist ideologies.
Assad's rule of Syria is a key pillar of Russia’s strategic influence in the Middle East and Moscow is very reluctant to let it go.
Since September 2014, the US-led coalition (including the RAF) has conducted air strikes on IS in Syria.
Russia launched an air campaign in Sept 2015 with the aim of "stabilising" the government after several defeats.
Hezbollah, the Shia Islamist movement in Lebanon, has sent thousands of fighters to support government forces.
Moscow said it would target only "terrorists", but activists said its strikes mainly hit Western-backed rebel groups.
Intense Russian air and missile strikes played a major role in the government's siege of rebel-held eastern Aleppo, which fell in Dec 2016.
Mr Assad acknowledged that “Russian intervention was the key factor that turned the balance of the war” in his favour.
'The Cold War never ended...Syria is a Russian-American conflict' says Bashar al-Assad
Shia power, Iran, is believed to be spending billions of dollars a year to bolster the Syrian government.
Iran has provided military advisers and subsidised weapons, as well as lines of credit and oil transfers.
Iran is also widely reported to have deployed hundreds of combat troops in Syria.
Assad is Iran's closest Arab ally and Syria is the main transit point for Iranian weapons shipments to the Hezbollah in Lebanon.
The US, says President Assad is responsible for widespread atrocities and must step down.
The US has provided only limited military assistance to "moderate" rebels.
The US was fearful that advanced weapons might end up in the hands of jihadists.
Sunni-ruled Saudi Arabia is seeking to counter the influence of its rival Iran.
Turkey is another staunch supporter of the rebels.
Turkey has tried to contain US-backed Kurdish Popular Protection Units (YPG) fighters who are battling IS in northern Syria.
The Turkish government accuse the YPG of being an extension of the banned Turkish Kurdistan Workers' Party (PKK).
