The United States hopes Arab countries involved in a diplomatic split with Qatar will soon present Doha a list of "reasonable and actionable" demands to move the crisis toward a resolution, US Secretary of State Rex Tillerson said on Wednesday.

Tillerson's comments came in a short statement a day after the State Department bluntly questioned the motives of Saudi Arabia and the United Arab Emirates in announcing their boycott of Qatar on June 5, saying it was "mystified" the Gulf states had not released their grievances.

It was Washington's strongest language yet on a dispute that erupted after the UAE, Saudi Arabia, Bahrain and Egypt cut diplomatic ties and transport links to isolate Qatar. The Arab countries accuse Qatar of funding terrorism, fomenting regional instability or cosying up to their enemy Iran. Qatar has denied the accusations.

"We understand a list of demands has been prepared and coordinated by the Saudis, Emiratis, Egyptians and Bahrainis," the Tillerson statement said. "We hope the list ... will soon be presented to Qatar and will be reasonable and actionable."

Tillerson also said the United States backs a Kuwaiti mediation effort aimed at resolving the crisis.

Asked about the Qatar issue at a later news conference, Tillerson said the United States wanted to achieve unity among the Gulf Cooperation Council countries so that they can focus on the fight against Daesh militants across the region.

"Our role has been to encourage the parties to get their issues on the table, clearly articulated, so that those issues can be addressed and some resolution process can get under way to bring this to a conclusion," Tillerson said.

Qatar hosts a vital US military base, Al Udeid, to which more than 11,000 US and coalition forces are deployed or assigned and from which more than 100 aircraft operate.

US President Donald Trump has taken a tough stance on Qatar, accusing it of being a "high level" sponsor of terrorism, but he has also offered help to the parties in the dispute to resolve their differences.