France has launched airstrikes on the IS Syrian stronghold of Raqqa in response to the Paris attacks, dropping 20 bombs on military targets.
World leaders had come to talk but first, there was a minute's silence for the victims of attacks in Paris and in the Turkish capital, Ankara.
Traditionally the G20 has been a forum primarily to discuss economic issues, but Turkish President Recep Tayyip Erdogan opened the summit by noting the strong link between security and the economy.
"The economy is the main interest of the G20 but it cannot be independent of political, social or cultural problems and it is never independent of human life."
Responsibility for the attacks in Paris has been claimed by the self-proclaimed IS, now one of the key players in the Syrian crisis.
Trying to bring peace to war-torn Syria has become a crucial goal of the G20 meeting as France retaliates to the Paris attacks by bombing key IS targets in Syria, including a command centre, munitions depot and training camp.
The United States wants to use the G20 summit to press its coalition partners to increase their commitments to its Middle East campaign.
US security advisor Ben Rhodes says there are no easy answers.
"We've been at war with ISIL for some time. For over more than a year now we've conducted thousands of airstrikes. We've provided arms to forces that are fighting them on the ground. But this is going to be a long-term campaign to disrupt and ultimately defeat ISIL and we're going to have to continue to redouble our efforts in partnership with allies like France."
President Barack Obama was seen in intense conversation with his Russian counterpart, Vladmir Putin, on the sidelines of the summit.
He is also facing questions domestically about how vulnerable the US might be to attacks of the kind that wrought havoc in Paris.
The concerns relate to reports that one or more of the attackers may have entered Europe as refugees.
President Obama has described the Paris attacks, like those carried out in the Turkish capital Ankara last month that killed more than 100 people, as attacks on the civilised world.
"Traditionally the G20 has been a forum primarily to discuss economic issues facing the globe ... (but) the sky has been darkened by the horrific attacks that took place in Paris just a day and a half ago."
Outside the G20 summit, among protesters on the streets of the host city Antalya, there's no welcome mat for President Obama and other visiting leaders.
Protest banners are calling the group of the world's largest economies an "imperialist criminal gang", and blame them for the bloodshed in Syria, Iraq and Afghanistan.
But British Prime Minister David Cameron has signalled a determination among G20 participants to achieve concrete action and, ultimately, peace.
"It's become even more clear that our safety and security depends on degrading and isolating and ultimately destroying ISIL, whether it's in Iraq or Syria. We are playing a huge role already in Iraq, others are now taking action in Syria which we support and enable but clearly we need to keep on making the case that we will be safer in the UK, in France, right across Europe, if we destroy this death cult once and for all."
