The United States and Qatar signed an agreement on combatting terrorism and its financing during a visit to Doha by US Secretary of State Rex Tillerson, his senior adviser R.C. Hammond told reporters.

"Qatar and the United States have signed a memorandum of understanding between the two counties outlining future efforts Qatar can take to fortify its fight against terrorism and actively address terrorism funding issues," he said.

"This is a hopeful step forward," Hammond added.

Tillerson told reporters in Doha earlier today that Qatar had "reasonable" views in the month-old diplomatic row with Arab neighbours and he was hopeful of progress towards a resolution.

Saudi Arabia, Bahrain, UAE and Egypt imposed sanctions last month, accusing Qatar of financing extremist groups and allying with Iran, the Gulf Arab states' arch-foe, something it denies.

"I am hopeful we can make some progress to begin to bring this to a point of resolution," Tillerson said alongside his Qatari counterpart Sheikh Mohammed bin Abdulrahman al-Thani.

"I think Qatar has been quite clear in its positions, and I think those have been very reasonable," Tillerson said.

The United States worries the crisis could impact its military and counter-terrorism operations and increase the regional influence of Iran, which has been supporting Qatar by allowing it to use air and sea links through its territory.

Qatar denies that it supports militant organisations and says the boycott is part of a campaign to rein in its independent foreign policy.