Separate talks between non-IS Syrian opposition groups are also underway in the Saudi capital, Riyadh, where they're being urged to close ranks and find common ground ahead of planned peace talks early next year.
The Gulf Cooperation Council brings together royalty and other dignitaries from Bahrain, Oman, Qatar, Saudi Arabia and the United Arab Emirates.
The conflict in Syria remains high on the agenda, which is why the summit coincides with separate talks aimed at eventually ending Syrian President Bashar al-Assad's grip on power.
About 100 representatives from Syria's fragmented political and armed opposition groups are gathering in Riyadh.
They're making an unprecedented bid for unity before potential negotiations with the Assad government.
Saudi Arabia's King Salman says his country remains committed to finding a solution in Syria.
"Regarding the Syrian crisis, the Kingdom of Saudi Arabia is hosting the Syrian opposition in order to help it find a political solution according to the Geneva accord. The international community has a joint responsibility in fighting terrorism and extremism, and the Kingdom of Saudi Arabia has exerted a lot of effort in this regard. We will continue our efforts with coordination and cooperation from our sisterly countries and friendly countries because our Islam rejects terrorism and extremism."
Recently, Iran warned that any talks held in parallel to international negotiations on Syria would violate the terms of last month's Vienna agreement.
Nevertheless the groups taking part in the Riyadh talks range from those fighting under the Free Syrian Army umbrella to factions that maintain ties with al-Qaeda.
They do not include representatives of IS.
But a Syrian Kurdish group that's taken control of areas in northern Syria claims it wasn't invited to the meeting, and there are reports Kurdish groups are organising their own gathering in Syria's Hasakah Province.
Meanwhile rebel fighters are among hundreds of people being evacuated from Syria's last insurgent-held area, Homs.
A ceasefire deal had been negotiated between opposition forces and the Assad government following a major Syrian army ground offensive to the north of Homs, backed by Russian air strikes.
Syria's Reconciliation Minister, Ali Haidar, says the deal reflects similar agreements being reached in other areas.
"All the areas surrounding the city of Homs have seen local agreements, including Homs and its suburbs. As a city, we've completed the work. Today, we consider this the last place in Homs. There are some promising starts, and as I've said in the past, it's parallel to the work on local reconciliation agreements in other cities, not just Homs."
Up to 15 buses have left the city, heading towards rebel-held areas of the northwest near the Turkish border.
It's an area the United States says needs a lot more attention in the battle against IS militants.
US Defense Secretary Ash Carter has renewed calls - for Sunni Arab nations in particular - to increase their efforts.
"Turkey must do more to control its often-porous border. Saudi Arabia and the Gulf States joined the air portion of the campaign in the early days - only the air part - but have since been preoccupied by the conflict in Yemen both in the air and on the ground. And just this past week I personally reached out to my counterparts in 40 countries around the world in the coalition and asked them to contribute more, in many cases contribute much more, to enhancing the fight against ISIL."
That conflict in Yemen has seen a bloody battle for regional power and influence play out between Iran and Saudi Arabia.
According to a recent New York Times article, Saudi Arabia's air force hasn't flown a mission against IS in Syria or Iraq since September.
Abdulkhaleq Abdulla is the Chairman of the Arab Council for Social Science, a Lebanon-based non-profit research centre.
He says the US needs to look at the broader regional picture before calling for increased military action from Arab stakeholders.
"Maybe there is a need for these two countries, Iran and Saudi Arabia, to agree on the end game. And I don't think that's coming any time soon. Where are we? We're left with nothing. We're left with no direction in the regional level. No direction on the international level. And then you come and ask the Arabs to join in. To do what?'
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