WASHINGTON (Reuters) - 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.

Buildings are seen on a coast line in Doha, Qatar, June 15, 2017. REUTERS/Naseem Zeitoon

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.

A U.S. official said Washington is urging Qatar to take steps to defuse the crisis, including signing on to proposals being drawn up the Treasury Department to strengthen controls against financing of militant groups.

But this official and a second U.S. official said it was inaccurate to single Qatar out, and that the Saudis, Emiratis and other Gulf states face similar challenges in countering terrorist financing.

Qatar’s foreign minister, who is expected to travel to Washington next week, said Doha would not negotiate with its neighbors to resolve the Gulf dispute unless they first lift the trade and travel boycott they imposed two weeks ago. He added that Doha still believed a solution was possible.

“Now that it has been more than two weeks since the embargo started, we are mystified that the Gulf states have not released to the public nor to the Qataris the details about the claims that they are making toward Qatar,” Nauert added.

There was no immediate comment from Riyadh or Abu Dhabi. Qatar’s ambassador to the United States, Meshal Hamad al-Thani, welcomed the State Department’s statement, tweeting: “We are confident in the ability of the U.S. to resolve this crisis”.

Qatar has denied accusations by its neighbors that it funds terrorism, foments regional instability or has cosied up to their enemy Iran.

The first U.S. official said the dispute is driven more by economic rivalries, historical tensions and the personal dynamics of Gulf leaders than by the specific demands the Saudis and Emiratis are making on Qatar.

The dispute has opened a rift among some of the main U.S. allies in the Middle East. Since the dispute erupted, Trump has taken a tougher stance against Qatar, while the State Department had sought to remain neutral.

Nauert said Tillerson had three phone calls and two in-person meetings with the Saudi foreign minister. Tillerson also spoke by phone three times with Qatar’s foreign minister and with the Qatari emir.

The UAE’s ambassador to the United States said last week a list of demands for Qatar was being compiled and would soon be handed to the United States.

He said they would broadly address support for terrorism, meddling in the internal affairs of these countries and attacks through Qatari-owned media platforms.

The Pentagon has said the boycott was hindering U.S. ability to plan for long-term operations in the region. Al Udeid is where command for the anti-Islamic State air campaign takes place.

Meanwhile, Qatar’s attorney general said on Tuesday his country has evidence that the hacking of Qatar’s state news agency was linked to countries that have severed ties with Doha.