President Barack Obama said Thursday that the United States and its Gulf allies were united in the war against the Islamic State extremist militia.

"We remain united in our fight to destroy ISIL or Daesh which is a threat to all of us," Obama told reporters, using two acronyms for the terrorist group, after talks with leaders of the six-nation Gulf Cooperation Council (GCC).

The regional conflicts in Syria, Yemen, Libya and Iraq as well as strengthening the US-led campaign against Islamic State were discussed during the meeting in Saudi capital Riyadh.

Obama said that the GCC countries pledged to increase their contributions to battling Islamic State, which controls large territory in Syria and neighbouring Iraq.

"We will continue to support Iraq as it liberates and stabilizes towns and cities from ISIL control," he said.

Obama, on the second day of a two-day visit to Saudi Arabia, sought to dispel GCC leaders' concerns about Washington's outreach to their regional rival Iran to reach a deal aimed at preventing Tehran from acquiring a nuclear weapon.

US support for the lifting of sanctions against Iran in the run-up to last year's landmark nuclear agreement has strained ties between the Washington and its traditional Gulf allies.

Obama on Thursday defended the Iran deal, saying it "cut off every single one of Iran's pathways to a nuclear weapon."

However, the US still has "serious concerns" about Iran's behaviour in the region, he said.

"I reaffirmed the policy of the United States to use all elements of our power to secure our core interests in the Gulf region and to deter and confront external aggression against our allies and our partners," Obama added.

The GCC comprises Saudi Arabia, Bahrain, the United Arab Emirates (UAE), Kuwait, Qatar and Oman.

The Sunni-ruled Gulf monarchies have repeatedly accused mostly Shiite Iran of meddling in their affairs and destabilizing the region.

Thursday's talks were the second in nearly a year between Obama and the Gulf leaders.

In May 2015, Obama hosted a GCC summit at the presidential retreat Camp David outside Washington and pledged an "iron-clad commitment" to Gulf allies, while briefing them on the details of the Iran nuclear deal.

Obama said Thursday that they reviewed "important progress" since that last meeting at Camp David and agreed to "build an even stronger partnership between our nations."

He emphasized the need to de-escalate regional conflicts in order to focus efforts on fighting Islamic State.

Obama arrived in Riyadh on Wednesday as part of an international tour before the end of his presidency in January that will also take him to Britain and Germany.

He met behind closed doors with King Salman, whose country and Iran back opposite warring sides in Yemen and Syria.

Saudi Arabia remains frustrated over Obama's decision to back down from airstrikes against Iran's ally, Syrian President Bashar al-Assad, after a chemical weapons attack on a rebel-held area of Damascus in 2013 that killed hundreds.

Washington has been critical of Saudi Arabia's human rights record, expressing concerns over the execution of dissidents including influential Shiite cleric Nimr al-Nimr in January.

Obama told reporters that he talked with Gulf leaders about human rights.

"Truce and lasting security," he said, depends on an economy that respects human rights. 

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