US Secretary of State Rex Tillerson will meet with his Qatari counterpart Tuesday in Washington, after the US department voiced confusion over the Saudi-backed embargo of the country.

Tillerson's meeting with Qatari Foreign Minister Sheikh Mohammed bin Abdulrahman Al Thani at the State Department also comes days after Doha dismissed a list of demands from Bahrain, Egypt, Saudi Arabia and the United Arab Emirates.

The ultimatum demands that Qatar comply with 13 points in return for an end to a three-week-old diplomatic and trade blockade of the country.

The four Arab governments announced earlier this month they were suspending all ties with Qatar, accusing it of support for extremist groups -- a claim Doha denies.

Tillerson has urged a diplomatic solution, and Washington has been pushing for a clear list of grievances that are "reasonable and actionable."

Signaling Washington's mounting frustration at Riyadh's role in the crisis, State Department spokeswoman Heather Nauert recently called on the parties to settle their differences.

And she called into question whether Qatar's alleged support for terrorism is the true cause of the crisis, or whether there is an underlying political dispute.

In addition to cutting diplomatic ties, Qatar's neighbors closed their airspace to Qatari carriers and blocked its only land border, vital for food imports.

Qatar is home to the largest US base in the region, Al-Udeid, and Bahrain is home to the US Navy's Fifth Fleet.

Source: AFP