The Gulf Cooperation Council's (GCC) annual summit was set to open in Riyadh on Sunday, with regional unity imperiled by a bitter row between Qatar and Saudi Arabia, which is mired in 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 neighbors.

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 the 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 UAE delegation to the Riyadh summit will be headed by Prime Minister and Vice-President Sheikh Mohammed bin Rashid al-Maktoum, who is also ruler of Dubai.

The GCC – set up in 1980 as a bulwark against larger neighbors Iran and Iraq – groups Saudi Arabia, UAE, Bahrain, Oman, Kuwait and Qatar. Kuwait's ties with Riyadh have also been strained over control of shared oilfields.