David Cameron is joining other world leaders in Turkey for a summit of the G20 group of leading economies, taking place in the shadow of the Paris terror attacks.
The murder of at least 129 in multiple attacks in the French capital has ensured the terrorist threat will dominate the annual gathering, which usually focuses on economic issues but has now become an opportunity for the international community to forge a response to atrocities committed by the Islamic State group.
The summit's host, Turkey's president Recep Tayyip Erdogan, urged fellow leaders to prioritise the issue, telling reporters: "We are now at a point where words end in the fight against terrorism. We are now at a stage where this should be put at the forefront."
Turkish media reported that four IS militants were killed after opening fire on soldiers in the east of the country on Saturday.
French president Francois Hollande himself has pulled out of the two-day summit being held in conditions of tight security in the Mediterranean resort of Antalya, as he prepares to address both houses of the French parliament in Versailles on Monday.
But Mr Cameron will be joined by other leaders already engaged in the struggle against IS - also known as Isis, Isil or Daesh - including US president Barack Obama, whose air force has been conducting strikes against the group in Syria and Iraq, and on Saturday targeted an Islamic State leader in Libya.
Also present will be Russian president Vladimir Putin, whose strikes against those he regards as terrorists in Syria are suspected by the West to be designed to shore up the rule of Bashar Assad rather than to defeat Islamic State. IS has claimed responsibility for downing a Russian airliner in Egypt's Sinai peninsula last month, killing 224.
The Kremlin announced that Mr Putin was expecting to meet Mr Cameron in Antalya for their first face-to-face talks since Russian air strikes began.
Even before Friday's attacks, the G20 agenda was expected to be topped by the search for a solution to the four-year civil war in Syria and to the waves of migration the conflict has unleashed across the Middle East and Mediterranean.
Turkey, which has a 500-mile border with Syria, has granted the US-led coalition access to its air bases to launch strikes against IS. Suicide bombers linked to the group have been blamed for the death of more than 100 people at a pro-Kurdish rally in the capital Ankara in October.
The summit comes a day after diplomats meeting in Vienna agreed on a timeline for a political transition in Syria, with a January 1 date for the start of talks between Assad's government and the opposition.
However, key details remained unresolved, including the question of what part Assad himself might play in any new settlement and a decision on which opposition groups should be shunned as terrorists. Only IS and al Qaida affiliates were specifically excluded from arrangements for proposed ceasefires.
The UN special envoy for Syria, Staffan de Mistura, is to begin immediate work on determining who should sit at the table in next year's talks, which are intended to establish a "credible, inclusive and non-sectarian" transitional government within six months ahead of UN-supervised elections within 18 months.
A British Government source said: "The G20 offers an opportunity for a number of the important players in Syria to be there 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 underestimate 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 Syrian refugees.
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.