- The regime's allies -
Moscow is Syrian President Bashar al-Assad's key ally, having stuck firmly by his regime since the conflict began in 2011. It has provided unconditional diplomatic backing and, since September 30, direct military support as well, carrying out at least 500 air strikes in Syria.
Russia, whose only Mediterranean military base is in Tartus on the Syrian coast, says it is fighting "terrorists", referring to all of Assad's opponents. But the West accuses it of primarily targeting moderate rebels rather than the Islamic State (IS) group.
Russian President Vladimir Putin pledged Tuesday to continue supporting Damascus militarily, while calling for a "political solution" involving all groups to end the war.
Tehran has also actively supported Syria's regime, although not as openly as Russia.
Iran does not officially acknowledge sending fighters to Syria, instead deploying members of its elite Revolutionary Guards as advisers. There are claims that they have been leading fighters from Afghanistan and Lebanon's Shiite movement Hezbollah, which Tehran denies.
According to a US official, at least 2,000 Iranian and Iran-backed fighters are bolstering Syrian army offensives on the ground in the country's north.
Tehran has not been invited to attend the Vienna meeting.
Baghdad and Damascus share a common enemy in IS. Since 2012, several Iraqi Shiite militias close to Tehran have sent fighters to Syria to support Assad.
- The regime's opponents -
THE UNITED STATES
US President Barack Obama has called Assad a "tyrant" and insisted on his departure since 2011. But Secretary of State John Kerry said Monday that Washington would discuss "options that could perhaps reignite the political process and bring about a political transition in Syria".
Washington has spearheaded an anti-IS coalition since September 2014, conducting air strikes in Syria and Iraq.
OTHER WESTERN COUNTRIES
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France and Australia, two coalition members who for months have been active in Iraq, recently started carrying out air strikes in Syria.
Britain, a coalition member that has carried out air strikes in Iraq, does not have parliament's approval to carry out air strikes in Syria. One of its drones however aimed at British targets in August.
Canada has also carried out strikes in Syria, but prime minister-elect Justin Trudeau said Tuesday his government would end them while remaining in the coalition.
French President Francois Hollande has warned Russia against strengthening Assad, "who is the problem, and cannot therefore be the solution".
Several European leaders, including German Chancellor Angela Merkel, have expressed openness to Assad taking part in a political process to end the conflict.
Coalition members Saudi Arabia and Qatar have said little about the nature and scope of their involvement in Syria, but both countries are widely believed to be among the biggest financiers and suppliers of materiel to anti-Assad rebels.
Qatar has made available its Udeid air base for Centcom, the US central military command for the Middle East and Central Asia.
Saudi Arabia has called for Assad's departure, but its foreign minister said Monday he could retain power until a transitional government was set up.
Turkey, a fervent opponent of the Syrian regime, has provided rebel groups with financial and logistical support since the beginning of the conflict.
Despite pressure from Washington and its allies, for a long time Ankara refused any military intervention against IS to avoid bolstering Syrian Kurds.
It recently accepted to open its southern airbase of Incirlik to American aircraft and drones targeting IS.
Ankara for the first time recently said a transition process with Assad might be possible.
Jordan has taken part in air strikes in Syria for a year, escalating them after IS claimed it had executed one of its pilots in February.
French military jets set off for Syria from a Jordanian airbase.