Gulf leaders have gathered gathered on Wednesday for an annual summit in the face of plunging oil revenues, the war in Yemen and pressure for peace in Syria.

The kings and emirs met in Saudi Arabia on Wednesday, as a bid to unify Syria's opposition took place elsewhere in Riyadh.

Qatar's Emir Sheikh Tamim bin Hamad Al Thani urged the Syrian opposition groups to close ranks and forge a common stand before planned peace talks expected by early next year.

"We urge the sides of the Syrian opposition to rise to the level of responsibility and to use this golden opportunity to unify their ranks and coordinate their steps beyond setting up a negotiating team," he said in a speech at the start of the summit.

The Gulf Cooperation Council (GCC) summit also comes days before warring factions from Yemen are to gather in Switzerland in an effort to end a costly war that has drawn in Gulf nations.

Talk to Al Jazeera: Lagarde's call to action - GCC should impose taxes

The GCC brings together Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates, whose leaders will hold two days of talks in the Saudi capital.

But despite the urgency of the challenges facing the six countries, analysts say the Gulf leaders will struggle to find common ground at the gathering.

"This summit comes as the Gulf is witnessing one of its most critical years," said Farea al-Muslimi, a visiting scholar at the Carnegie Middle East Center.

He sees "internal disagreement" among the Gulf states as they face complicated economic and security challenges.

These include greater worries about Iran after a July deal that will ease sanctions on the predominantly Shia nation, including its oil sector, in return for restrictions on its nuclear facilities.

Iran and Sunni-dominated Saudi Arabia are rivals for regional influence in Yemen, Syria and elsewhere.

"The main challenge facing the GCC summit is, as usual, trying to ensure a united front on the major strategic challenges in the region," said Neil Partrick, author of a forthcoming book on Saudi foreign policy.

The summit coincides with Saudi Arabia's hosting - also on Wednesday and Thursday - talks it hopes could help ease out Syria's President Bashar al-Assad.

About 100 representatives from Syria's fragmented political and armed opposition groups are gathering in Riyadh in an unprecedented bid for unity before potential negotiations with Assad's regime.

Yemen turmoil

For more than eight months, Gulf military forces have been fighting in Yemen to support President Abd-Rabbu Mansour Hadi's government alongside an array of local anti-rebel forces.

The coalition has been trying to push Shia Houthi rebels and allied troops from territory they occupied in Yemen.

Armed groups like al-Qaeda in the Arabian Peninsula (AQAP) have taken advantage of the chaos to expand their presence.

Oman is the only GCC state not part of the coalition, but has mediated and provided a neutral venue for talks.

According to the United Nations envoy to Yemen, another attempt at peace talks will start on December 15, after earlier efforts collapsed.

Inside Story: Oil price wars - competition or control?

The GCC was founded to more deeply integrate the Gulf countries but Jane Kinninmont, of the Middle East and North Africa programme at the Chatham House think-tank in London,  said the emphasis over the past few years has been on intelligence and security cooperation.

Low oil prices "should focus the minds of GCC leaders" on economic integration and joint infrastructure development, in line with the wishes of many Gulf citizens, she said.

Crude prices have more than halved since early 2014 and the IMF has projected a $275bn drop in export revenues this year for the resource-dependent Gulf economies.

Source: Agencies