The leaders of the world’s 20 main industrial and emerging economies observed a minute of silence at the opening of the summit, which is being held amid massive security precautions.

Although the first few sessions of the summit were devoted to the world economy and guarding against climate change, in the corridors outside the meeting halls the main points of discussion were terrorism and the fight against the Islamic State jihadist group that has claimed responsibility for the Paris attacks.

Early in the morning, before the start of the summit, U.S. President Barack Obama expressed sorrow over the attacks, calling them “an attack on the civilized world.”

Turkish President and summit host Recep Tayyip Erdogan emphasized that what occurred in France shows that the world “cannot ignore” the relationship between the economy and politics.

The main interest of the G20, the economy, “is not at the margin of social, political and human life,” said the Turkish leader.

Shortly thereafter, Obama held an informal meeting – not initially scheduled – with his Russian counterpart, Vladimir Putin, to discuss how to end the war in Syria and defeat the IS.

According to the White House, the two leaders agreed to begin a transition in Syria, although Russian officials emphasized that “differences” remain between them regarding the tactics to use to eliminate the IS.

Since September, Russia has been mounting airstrikes on the Syrian opposition – both jihadists and more moderate forces – in support of the regime of President Bashar Al-Assad.

Those attacks against the Syrian moderate groups were harshly criticized on Sunday by European Council President Donald Tusk, who urged the G20 leaders to focus their efforts against the IS since “they are the real enemy of the free world, not the moderate (Syrian) opposition.”

He went on to say that “there is no better forum (than the G20) to talk about the financing of terrorism. The terrorist networks cannot plan or operate without money that moves through the financial system of many countries.”

German Chancellor Angela Merkel, meanwhile, said on Sunday that the Antalya summit will send a “decisive” message after the Paris attacks.

“We are stronger than any form of terrorism,” said the German leader, insisting that “we owe it to the victims and their families, but also to our own security and to the many innocent refugees fleeing from war and terrorism” to determine who carried out – and collaborated with – the Paris attacks.

European Commission President Jean-Claude Juncker urged Europeans not to confuse Middle Eastern refugees flooding into Europe with the terrorists, noting that the terrorists are “criminals” but the refugees “are fleeing from the philosophy and mentality that inspire” acts like those perpetrated in the French capital.

He added, however, that attacks of this kind endanger the continuity of the Schengen zone for the free circulation of people around the European Community, going on to say that greater measures must be taken to strengthen the EC’s external borders.

The G20 leaders held a working dinner on Sunday at which the scheduled topic of discussion was the terrorist threat and the refugee crisis.