The US state department says it is "mystified" that Gulf states have not released details of the claims they are making against Qatar, more than two weeks after they imposed a blockade on the country and cut ties with it.
Heather Nauert, spokesperson for the state department, said in a press briefing on Tuesday that the more time goes by, "the more doubt is raised" about the anti-Qatar measures imposed by Saudi Arabia, the United Arab Emirates (UAE) and their allies.
"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," she said, referring to the Gulf Cooperation Council.
The three Gulf nations, along with Egypt, severed diplomatic ties and cut off sea and air links with Qatar on June 5, accusing it of supporting "terrorism" - a charge Doha strongly denies.
They ordered Qatari nationals to leave their countries, and also urged their citizens to return to their respective nations, disrupting the lives of thousands in the region and restricting their freedom of movement.
MARWAN BISHARA, AL JAZEERA'S SENIOR POLITICAL ANALYST:
There are three levels to understanding the statement by the US state department, which I think is incredibly important.
Firstly, it was the language used, which was quite harsh and direct. There was a sense of reprimand, almost a slap in the face for the Emiratis and the Saudis for not coming forward with something substantial.
Secondly, this came out publicly - this was not harsh language written in a letter sent to the foreign ministry in Saudi Arabia or the UAE. This was in a press conference, for the whole world to hear.
And thirdly, it's a sense of impatience, bordering on a feeling of insult by the Saudis and the Emiratis. There is a sense that US Secretary of State Rex Tillerson is telling them, 'Are you toying with us? Have you played the American president about Qatar supporting terrorism, hanging him out to dry for already going public and accusing Qatar of something that does not exist'?
Sanctions also disrupted food and other imports into Qatar.
Qatar's Foreign Minister Sheikh Mohammed bin Abdulrahman Al Thani said Doha would not negotiate with its neighbours to resolve the dispute unless they first lift the trade and travel restrictions.
Qatar hosts the largest US military airbase in the Middle East, Al-Udeid, where more than 11,000 US and coalition forces are deployed or assigned and from which more than 100 aircraft operate.
Over the past two weeks, US President Donald Trump has taken a tougher stance against Qatar, while the State Department and the Pentagon had previously sought to remain neutral in the Gulf crisis.
In remarks made on June 9, Trump said that "the nation of Qatar has historically been a funder of terrorism at a very high level", without, however, providing any evidence.
Al Jazeera's James Bays, reporting from Washington, DC, said that the state department's statement on Tuesday marked a change in US policy.
"Two weeks into the Gulf crisis, the state department is effectively changing sides, criticising those blockading Qatar," he said.
Bays said that the statement was "almost certainly cleared in advance" with the Pentagon and with the White House.
"But with this administration, you have to ask one question: is the president himself on board with the new policy?" he added.
"It also seems highly likely some of those countries who supported the blockade will now be lobbying President Trump to overturn this new position."
Qatar's foreign minister is expected to travel to Washington next week to hold talks about the ongoing crisis.
Marwan Bishara, Al Jazeera's senior political analyst, said the timing of the statement by the state department before the Qatari foreign minister's visit was "very important".
"The hosting of the foreign minister [is very significant too]," he said.
"Do you really, on the one hand, accuse a country of 'supporting terror on a high level', and on the other have your main military base in the whole region that is fighting your war on terror in that country?
"And now, hosting its own foreign minister in your capital? Does any of that square?"