Leaders of Gulf Arab countries, including those boycotting Qatar, met on Sunday in Saudi Arabia's capital for a regional summit, a gathering that Qatar's ruling emir choose not to attend amid the dispute.

The absence of Qatar's Sheikh Tamim bin Hamad Al Thani, as well as Qatar's recent decision to withdraw from the Saudi-dominated OPEC oil cartel, underlined the simmering crisis facing the Gulf Cooperation Council.

Despite mediation efforts by Kuwait, the gap remains between others in the six-nation GCC, an alliance formed in 1981 in part to offer a counterbalance to Iran, and Qatar — countries that are all American allies. The United States, which under President Donald Trump withdrew from the Iran nuclear deal, also has been unable to mend relations even as it wants a unified front for its high-pressure campaign targeting Tehran.

Saudi King Salman greeted GCC leaders Sunday as they descended down a golden escalator at a Riyadh air base and never mentioned Qatar in his remarks to the summit. The king instead spoke about the need to counter Iran, as well as the kingdom's ongoing war in Yemen.

His son, Crown Prince Mohammed bin Salman, who has been linked to the slaying and dismembering of Washington Post columnist Jamal Khashoggi at the hands of Saudi agents in October in Istanbul, sat prominently behind his father.

Kuwait's ruler, the 89-year-old Sheikh Sabah Al Ahmad Al Sabah, did obliquely discuss Qatar, warning of internal divisions threatening the GCC.

"We are all concerned about what's going on and that's why we have to embody our unity . because we face a lot of challenges," Sheikh Sabah said.

Qatar's Sheikh Tamim sent Sultan bin Saad al-Muraikhi, minister of state for foreign affairs, to represent the country. Al-Muraikhi offered no comments during the open session of the GCC meeting. The sheikh had attended last year's GCC meeting in Kuwait, where nations boycotting Qatar sent lower-level representatives.

Bahrain's Foreign Minister Khalid Al Khalifa tweeted that Sheikh Tamim should have "been there at the summit."

In June 2017, Bahrain, Egypt, Saudi Arabia and the United Arab Emirates cut diplomatic ties to Qatar. They also launched an economic boycott, stopping Qatar Airways flights from using their airspace, closing the country's sole land border with Saudi Arabia and blocking its ships from using their ports.

They say the crisis stems from Qatar's support for extremist groups in the region, charges denied by Doha. The four nations also have pointed to Qatar's close relationship with Iran. Qatar restored full diplomatic ties to Iran amid the dispute.

Qatar's natural gas reserves have made the Arabian Peninsula country fantastically wealthy, sparking its successful bid for the 2022 FIFA World Cup. Qatar also hosts the al-Udeid Air Base, the home of the forward headquarters of the U.S. military's Central Command and about 10,000 U.S. troops.

The U.S. deputy assistant secretary for Arabian Gulf affairs, Timothy Lenderking, said Washington wants to see Gulf unity and ties with Qatar repaired soon.

"GCC unity's been a really important pillar for U.S. engagement with this region, with the Gulf, with the Middle East. We'd like to see that unity restored," he said on the sideline of a security summit on Yemen in Abu Dhabi on Sunday.

"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 capitalize on economic linkages that can add development to the region and that can help the countries bind together," he said.