Leaders of the G20 group of major world economies stood for a minute's silence in honour of victims of terror in Paris and Ankara as their two-day summit got under way in Turkey.
French President Francois Hollande was absent from the gathering to lead his country's response to the slaughter in Paris.
David Cameron joined fellow leaders including US President Barack Obama and Russian President Vladimir Putin in bowing their heads for a moment to remember the 129 victims of Friday's gun and bomb attacks, as well as the 102 killed by two blasts at a demonstration in the Turkish capital on October 10.
After meeting the summit's host, Turkish President Recep Tayyip Erdogan, Mr Obama vowed that the US would "redouble efforts" to prevent further attacks by Islamic State, which has claimed responsibility for the Paris atrocities, has been blamed for the Ankara massacre, and is believed by Britain to be guilty of downing a Russian airliner over the Sinai peninsula in Egypt last month.
"Traditionally the G20 has been a forum primarily to discuss economic issues facing the globe," said the US president. "(But) the sky has been darkened by the horrific attacks that took place in Paris just a day and a half ago.
"The United States and its allies will redouble efforts to find a peaceful solution in Syria and prevent Islamic State militants from perpetrating attacks like those in Paris."
The fight against terror is set to overshadow the economic issues on the official agenda for the summit, taking place in the beach resort of Antalya, just 300 miles from the Syrian border. And issues like migration and the search for peace in Syria will be seen through the prism of the violence in Paris.
Following the discovery near the body of one of the killers of a passport belonging to a Syrian refugee who entered the EU through Greece, questions were being raised about the future of the border-free travel arrangements in place in much of continental Europe.
European Commission president Jean-Claude Juncker urged member states not to ditch the EU's refugee policy, saying: "We should not mix the different categories of people coming to Europe. The one responsible for the attacks in Paris ... he is a criminal and not a refugee and not an asylum seeker.
"I would invite those in Europe who try to change the migration agenda we have adopted - I would like to remind them to be serious about this and not to give in to these basic reactions that I do not like. I see the difficulty but I don't see the need to change our general approach."
Mr Cameron will meet Mr Putin on Monday for the first time since Moscow launched air strikes in Syria in September, and is likely to call on the Russian president to ensure that missions are targeted at IS, rather than at the more moderate opposition groups fighting the regime of Bashar Assad.
Moscow and the West are at loggerheads over Mr Putin's support for Assad. A meeting of foreign ministers in Vienna on Saturday agreed a two-year timeline towards the election of a new government in Damascus, but key details remained unresolved, including what part Assad 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.
With the attention of the West increasingly focused on the elimination of the threat from IS - also known as Isis, Isil or Daesh - Mr Putin can be expected to urge the Prime Minister to give up Britain's insistence that Assad can have no long-term role in a future administration.
Mr Putin shook hands with President Obama as the pair talked for around 30 minutes on the sidelines of a working lunch, but they are not due to hold a formal meeting.
Washington is known to be concerned that US jets conducting strikes against IS strongholds in Syria do not come into conflict with Russian planes - a danger which is also believed to be a factor in Mr Cameron's decision not to press ahead so far with a Commons vote on extending RAF missions into Syria.
European Council president Donald Tusk called for a united front against IS. "It should be our common aim to co-ordinate our actions against Daesh, and for sure the co-operation between the United States and Russia is a crucial one," he said.
Mr Erdogan has said that concerted action in the fight against terrorism should be "put at the forefront".
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. Turkish media reported that four IS militants were killed after opening fire on soldiers in the east of the country on Saturday.