World powers are meeting in Vienna in the latest round of discussions to bring an end to civil war in Syria.
Hosted by the US, the talks in the Austrian capital will bring together Western nations including the UK - represented by Foreign Secretary Philip Hammond - with ministers from Russia and regional powers Iran and Saudi Arabia.
No representatives of the Syrian opposition are expected to take part in the official discussions, which will focus on determining which rebel groups should be regarded as "terrorists" and which should be involved in negotiations on the transition to a new political settlement following the departure of President Bashar Assad.
Syria is also expected to dominate the annual gathering of the G20 group of nations opening in Turkey on Sunday, where Prime Minister David Cameron is due to hold bilateral talks with Russia's President Vladimir Putin - a staunch ally of Assad - whose launch of air strikes against what he regards as terrorist groups are widely seen as an effort to bolster the Syrian leader.
Moscow's proposals for elections in Syria following an 18-month constitutional reform process have received a frosty reception in the West because they do not guarantee Mr Assad's departure from power. Although Mr Hammond has previously signalled a willingness for Mr Assad to remain in place for at least part of the transition process, London remains insistent that he has no long-term future in Syria's administration.
The first round of talks in Vienna last month were regarded as a breakthrough because of the presence in the same room of Iran and Saudi Arabia, which are engaged in a long-running struggle for influence in the region. Tehran has been one of the Assad regime's strongest backers.
The new talks take place against the backdrop of setbacks for the Islamic State terror group, which has lost control of the strategic town of Sinjar in Iraq, and saw British militant Mohammed Emwazi - who fronted many of its beheading videos - apparently killed in a US drone strike in its Syrian stronghold of Raqqa.
Some 17 countries and three international bodies are expected in Vienna, where talks will focus on progress towards political transition in Syria, achieving a reduction in violence, building trust and improving humanitarian access.
But US Secretary of State John Kerry, who is chairing the talks, has cautioned against any expectations of a deal, stating that the process was not "on the threshold of a comprehensive agreement". Ambitions are instead directed at agreeing a structure for political transition and moving forwards to future talks which will include representatives of the different sides in the Syrian conflict.
Syria will be high on the agenda in the Turkish resort of Antalya when Mr Cameron joins leaders of other world powers at the two-day G20 summit, which will also focus on the migration crisis and the international threat from terrorism.
A British Government source said: "The G20 offers an opportunity for a number of the important players in Syria to be their together and to have some discussions about the way forward, following on from the foreign ministers' discussions in Vienna.
"There's still a lot to do to bring the differing sides and positions together.
"Vienna has been a step forward, but we don't under-estimate how far there is still to go. That is going to take political leadership, will and some real momentum."
Mr Cameron earlier this week announced £275 million in UK aid for Turkey over the next two years, to help it cope with an influx of around two million refugees from neighbouring Syria.
He will push for progress on initiatives to help refugees earn a livelihood while living in camps in neighbouring countries, ahead of their eventual return to Syria.
The Kremlin confirmed that Mr Putin was expecting to meet Mr Cameron for the first time since Russian air strikes began, and since last month's crash of a Russian airliner in Egypt, which Britain believes was almost certainly caused by terrorism.