* Gulf Cooperation Council holds one-day annual summit
* Overshadowed by continued row between Qatar and Saudis
* Qatar has not said who will represent it at summit
* U.S. urges Gulf states to mend fences (Adds comments by US official, UAE representation)
By Stephen Kalin
RIYADH, Dec 9 (Reuters) - The Gulf Cooperation Council's (GCC) annual summit was set to open in Riyadh on Sunday, with regional unity imperilled 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 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 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 neighbours 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.
WASHINGTON URGES UNITY
Following global outrage over the Oct. 2 murder of Khashoggi at Saudi Arabia's Istanbul consulate, Washington has increased pressure on Riyadh to end the nearly four-year-old Yemen war that has pushed that country to the brink of famine and to restore ties with Qatar for a united Gulf front against Iran.
A U.S. State Department official said on Sunday that Washington would continue to support the Saudi-led military coalition in Yemen and urged Gulf states to mend fences, including to enable a proposed new Middle East security alliance that would include the Gulf bloc, Egypt and Jordan.
"We'd like to see that unity restored, not on our terms, but on terms of the countries that are involved," Timothy Lenderking, Deputy Assistant Secretary for Arabian Gulf Affairs, told reporters at a security forum in the UAE capital Abu Dhabi.
"It's important not only for the GCC to be a strong bulwark against Iranian influence in the Arabian Peninsula but also to allow us to capitalise on economic linkages that can add development to the region and help the countries bind together."
Qatar's exit from OPEC after 57 years 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 is unlikely at Sunday's Riyadh summit.
While the boycotting states have said the row is not a priority for them and that the GCC remains valid, Doha has said the dispute harms 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. (Additional reporting by Alexander Cornwell in Abu Dhabi Editing by Ghaida Ghantous and Gareth Jones)
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