The U.S. State Department said on Tuesday it was "mystified" that Gulf Arab states had not released to the public or to Qatar details of the grievances that prompted their economic and political boycott of the country.

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."

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"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 five-nation Gulf Cooperation Council.

Qatar hosts a vital U.S. military base which more than 11,000 U.S. and coalition forces are deployed or assigned to and from which more than 100 aircraft operate. The dispute between Doha and nations led by Saudi Arabia has yet to shake that partnership, though cracks are showing in responses from President Donald Trump and his administration.

Trump called on Qatar to stop funding of groups that commit terrorism, saying the Gulf nation had historically done so "at a very high level."

"No civilized nation can tolerate this violence or allow this wicked ideology to spread on its shores," Trump told reporters two weeks ago at the White House.

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

Qatar's foreign minister 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 was 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.

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

The dispute has opened a rift among some of the main U.S. allies in the Middle East. Over the last two weeks U.S. President Donald Trump has taken a tougher stance against Qatar, while the State Department had previously sought to remain neutral.