The U.S. State Department bluntly questioned on Tuesday the motives of Saudi Arabia and the United Arab Emirates for their boycott of Doha, saying it was "mystified" the Gulf states had not released their grievances over Qatar.

In Washington's strongest language yet on the Gulf dispute, the State Department said the more time goes by, "the more doubt is raised about the actions taken by Saudi Arabia and the UAE."

"At this point, we are left with one simple question: Were the actions really about their concerns regarding Qatar's alleged support for terrorism or were they about the long-simmering grievances between and among the GCC countries," State Department spokeswoman Heather Nauert said, referring to the six-nation Gulf Cooperation Council.

The State Department's comments came in contrast to the language taken by U.S. President Donald Trump who has accused Qatar of being a "high level" sponsor of terrorism.

Saudi Arabia, the United Arab Emirates and Qatar are key American allies.

The fact the State Department bluntly questioned Riyadh and Abu Dhabi's actions in public suggests Washington was keen for the parties to end the dispute.

"We've just said to the parties involved: Let's finish this. Let's get this going," Nauert said.

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

The United Arab Emirates, which along with Saudi Arabia, Egypt and Bahrain imposed the measures to isolate Qatar, has said the sanctions could last for years unless Doha accepted demands that the Arab powers plan to reveal in coming days.

The State Department, headed by Secretary of State Rex Tillerson, was encouraging "all sides to de-escalate tensions and engage in constructive dialogue," Nauert said.