The Gulf Cooperation Council's (GCC) annual summit was set to open in Riyadh on Sunday, with regional unity imperilled by a bitter row with Qatar and the host, Saudi Arabia, facing a diplomatic crisis over the murder of journalist Jamal Khashoggi.
The one-day annual gathering of leaders from the six member states is expected to focus on security issues, including the Yemen war and Iran's regional activities, and may touch on oil politics and a protracted boycott of Qatar by some of its neighbours.
Saudi Arabia, the United Arab Emirates, Bahrain and non-GCC member Egypt cut diplomatic and economic ties with Qatar in June 2017 over allegations of supporting terrorism.
Qatar, which last week abruptly announced it was withdrawing from oil exporters group OPEC, denies the charges and says the boycott aims to curtail its sovereignty.
The Saudi king has invited Qatar's emir to the summit, but Doha has not said what level of representation it would send. The emir attended last year's gathering in Kuwait, while Saudi Arabia, the UAE and Bahrain sent more junior officials.
The GCC -- set up in 1980 as a bulwark against larger neighbours Iran and Iraq -- groups Saudi Arabia, UAE, Bahrain, Oman, Qatar and Kuwait, whose ties with Riyadh have also been strained over control of shared oilfields.
Saudi Arabia has resisted renewed U.S. pressure to end the Qatar row after the Oct. 2 murder of Khashoggi at the kingdom's Istanbul consulate drew global condemnation and exposed Riyadh's domestic crackdown on dissent and activities in the region.
Rights group Amnesty International called on GCC states to release peaceful dissidents in the region, where governments have shown little tolerance for open dissent or criticism of rulers.
"Gulf leaders can no longer operate on the assumption that they have a carte blanche to treat their citizens like criminals whenever they express dissent without fear of any international repercussions," said Heba Morayef, Middle East Director of Campaigns.
The United States has increased pressure on Riyadh following Kashoggi's killing to end the Yemen war and mend fences with Qatar as Washington wants Gulf states to present a united front against Iran.
Qatar's exit from OPEC after 57 year to focus on gas appeared to be a swipe at the bloc's de facto leader Saudi Arabia. The move has deepened the sense among diplomats and analysts that any prospect for a near-term resolution to the dispute was unlikely at the Riyadh summit.
While the boycotting states have said the row is not a priority for them and that the GCC remained valid, Doha has said the dispute harmed regional security by weakening the bloc.
Relations have also soured between Saudi Arabia and Kuwait over oil production from two jointly-run oilfields in the so-called Neutral Zone after talks in September failed to move the two countries closer to a deal.