Russia last week launched air strikes in Syria in its biggest Middle East intervention in decades. The Russian involvement plunged the four-year-old Syrian civil war into a volatile new phase, as President Vladimir Putin moves forcefully to stake out Moscow's influence in the unstable region.

The strikes have also raised the alarm of the United States and Turkey after Russian fighter jets entered Turkish air space in two separate incidents at the weekend. That prompted Ankara to summon the Russian ambassador twice to protest both violations.

With Moscow entering the fray, the Syrian war now involves multiple countries with overlapping agendas. Here is a look at the key players in the conflict and where they stand.

SYRIA

Syria's conflict grew out of protests against President Bashar al Assad's rule in early 2011. The protests were put down by force before increasingly turning violent and drawing international concern. In September last year, the US began leading an international coalition in Syria to conduct air strikes on Islamic State in Iraq and Syria (ISIS) militants and other extremist groups.

Since the Syrian civil war began, about 250,000 people have died and nearly half of Syria's prewar population of 23 million has fled, with many thousands attempting to reach Europe.

RUSSIA

Moscow says it is joining the war in Syria to back the West in its battle against ISIS militants and other terrorist groups. Since entering the war, it has carried out dozens of attacks daily on ISIS targets within Syria. It says it fears a blowback of thousands of young Russians who have joined ISIS coming home to carry out attacks.

Russia is also interested in supporting Assad from US-backed moderate rebels who have been working to overthrow the Syrian leader. Assad has been Russia's only persistent ally in the Middle East for decades.

Russia's support for Assad has come in the form of blocking UN resolutions critical of him at the UN Security Council, while supplying weapons to the Syrian military despite facing international criticism.

While Moscow has said its intervention targets the hardline ISIS fighters who control much of eastern and northern Syria, many of the Russian strikes so far have hit rival insurgent forces opposed to Assad.

Experts say Russia's involvement in Syria could also run much deeper. They say Russia's recent military build-up in Syria aims not only to boost Assad's embattled regime but also to send a strong signal to the West. According to Daragh McDowell, an analyst with the Verisk Maplecroft consultancy, there is little doubt that the build-up is "aimed at forcing the US and the West to re-engage with Moscow" in its future vision for Syria.

Senior officials and diplomats have also told Reuters they believe Russia's aim is to use its intervention in Syria partly to pressure the West to tacitly accept Russia's 2014 annexation of Crimea and gradually lift the sanctions imposed by the West over the annexation.

UNITED STATES

The US is using Syrian airspace to lead a campaign of air strikes against ISIS backed by its allies which include Australia, France, Britain, Turkey and Saudi Arabia, among others. While both Moscow and Washington say their enemy is ISIS, the US believes that Russian ally Assad's continued presence in Syria makes the country's situation worse. Washington blames him for attacks on civilians that have radicalised the opposition and insists that he has no place in a post-war settlement in Syria.

Instead, the US strikes are aimed at supporting the more moderate of the rebel factions in Syria which oppose Assad's government, as well as ISIS and other extremist groups.

TURKEY

Turkey and Russia have long been at loggerheads over the Syrian conflict, with Ankara seeking the overthrow of Assad while tacitly supporting the rebel forces in Syria.

Turkey has also stepped up its role in the US-led coalition as violence in Syria and Iraq has increasingly been spilling over its borders.

Turkish president Recep Tayyip Erdogan has also said that his country has borne the brunt of the fallout of the Syrian war, as a result of the exodus of refugees into neighbouring countries, including Turkey. Ankara says it is now hosting almost two million Syrians.

Turkey in recent days has also stepped up its criticism of Russia over the Syrian strikes which have been conducted in violation of its air space. Russian defence ministry spokesman said on Wednesday (Oct 7) that Moscow could implement US proposals that are aimed at coordinating Moscow's strikes in Syria with the Washington-led coalition.

IRAN

Syrian ally Iran has been providing military support, weapons and financial aid to Damascus since the Syrian war began. The bloodshed in Syria, is also seen by analysts as part of a wider struggle for regional supremacy between Shi'ite Iran and Sunni Muslim Saudi Arabia. The crisis has also aggravated sectarian anger across the Middle East and drawn religiously motivated foreign fighters to both sides.

SAUDI ARABIA

Riyadh, along with Turkey and other Gulf states, is a main supporter of rebels fighting Assad. But the Saudi government is also worried about the rise of jihadist groups such as ISIS among the opposition. As such, the government has decreed long prison terms for anyone who supports ISIS, whose sympathisers have killed dozens in attacks in the kingdom this year.

SOURCE: AGENCE FRANCE-PRESSE, NEW YORK TIMES, BBC, REUTERS, CNN