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US Defence Secretary Jim Mattis reaffirmed America's strategic security partnership with Qatar on Thursday, the Pentagon said, amid a diplomatic crisis in the Gulf.

Mattis, who spoke with his Qatari counterpart Khaled bin Mohammed al-Attiyah by phone, discussed the status of operations against the Islamic State group.

Qatar hosts a vital US-led command center at the al-Udeid air base, where the anti-IS coalition launches raids against the jihadists.

"Secretary Mattis and Minister al-Attiyah affirmed their commitment to continued US-Qatar cooperation and deepening their strategic partnership," a Pentagon readout of the conversation stated.

Saudi Arabia is leading a four-country blockade of Qatar in the region's biggest crisis in years.

Mattis stressed the importance of de-escalating tensions "so all partners in the Gulf region can focus on next steps in meeting common goals", the readout stated.

Tillerson in Kuwait

Despite numerous US appeals for Saudi Arabia, Bahrain, the United Arab Emirates and Egypt to resolve their issues with Qatar on their own, the State Department said on Thursday that Secretary of State Rex Tillerson would visit the region next week to discuss the row.

Spokesperson Heather Nauert said Tillerson would visit Kuwait, which is acting as mediator in the crisis, on Monday after stops in Ukraine and Turkey.

She said Tillerson would meet with Kuwaiti officials, but his presence in the region leaves open the possibility that he may try to shuttle between the neighbouring countries to forge a resolution.

The US has been supporting Kuwait's mediation efforts, but Tillerson's trip will mark a new level of US involvement in trying to broker a resolution.

Earlier on Thursday, Nauert warned that the crisis over Qatar might not be quickly resolved.

"We've become increasingly concerned that that dispute is at an impasse at this point," she said.

"We believe that this could potentially drag on for weeks; it could drag on for months; it could possibly even intensify."

She did not specify what type of escalation the US fears, but she said Tillerson remains in close contact with the countries involved.

The four nations cut off diplomatic ties to Qatar a month ago, accusing the gas-rich nation of funding terrorism and spreading unrest, charges Doha has vehemently denied.

Tillerson had previously said that the 13 demands imposed by the Saudi-led bloc, which Qatar has refused to comply with, were "very difficult" to meet.