The US says it is “mystified” as to why Saudi Arabia and its Gulf allies have not revealed details to back up their claims that Qatar supports terror groups.
Saudi Arabia, Bahrain, Egypt and United Arab Emirates cut ties with the Qataris and imposed an economic blockade earlier this month to isolate it.
There are warnings that their sanctions could last for years.
Qatar denies the allegations and in a sign it is shifting its tone, the Trump administration is also calling on the nations to come up with a list of demands for the Qataris to consider as it tries to settle the embargo row.
The UAE has said the Arab nations plan to reveal their demands in the coming days and has urged Doha to accept them.
Saudi Arabia and the UAE are two of America’s closest allies. During a trip to Riyadh last month, President Donald Trump praised the Saudis.
And in Qatar, there is a US military base housing more than 11,000 US and coalition forces, from where more than 100 aircraft operate.
US state department spokeswoman Heather Nauert has urged all parties to settle their differences.
And she called into question whether Qatar’s alleged support for extremism is the true cause of the crisis, or whether it is due to underlying political tensions.
She said: “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 Qataris nor to the public the details about the claims they are making toward Qatar.
“The more that 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?” she asked, referring to the Gulf Cooperation Council, which includes both Doha and Riyadh.
Meanwhile, Turkish troops have arrived in Doha for joint military exercises in a show of support for the Qataris. Turkey has a military base in the country.
Iran has been sending fruit and vegetables to Qatar after the Gulf nations cut trade links with the emirate.