Jeddah, Saudi Arabia: US Secretary of State Rex Tillerson held talks Wednesday with four Arab states boycotting Qatar as part of a round of intense shuttle diplomacy aimed at resolving the regional crisis.

Tillerson flew into Saudi Arabia where he met King Salman, whose country is leading a four-state alliance that has cut ties with Qatar over accusations that it supports extremism.

He then began talks with the foreign ministers of Saudi Arabia, Egypt, the United Arab Emirates and Bahrain in an attempt to mend fences between the crucial US allies.

In a setback to his efforts, the four Arab states on Tuesday quickly dismissed a counter-terrorism deal signed between Qatar and the United States as "insufficient".

The crisis has presented Tillerson, well known in the Gulf from his former role as chief of energy giant ExxonMobil, with his first big challenge as Washington's top diplomat.

While President Donald Trump welcomed the Arab states decision to sever air and land links to their gas-rich neighbour, the US State Department has taken a more neutral position and Tillerson is seeking to broker a diplomatic solution.

Tillerson, who is spending much of this week in the Gulf seeking to end the dispute, passed on "greetings from President Trump" at his audience with King Salman at the royal court in Jeddah.

"He wanted to ensure that I extend to you his warmest regards," Tillerson told the Saudi ruler.

The United States and its Western allies have vast economic and political interests in the Gulf, which pumps one fifth of the world's oil supplies.

While Saudi Arabia is a key US ally, Qatar is home to the US military's largest air base in the region, Al-Udeid. Rival Bahrain houses the US Navy Fifth Fleet.

On Tuesday, after a stop in regional mediator Kuwait, Tillerson travelled to Doha where he described Qatar as being "reasonable" in its dispute with the four states.

He also signed a deal which he said "lays out a series of steps the two countries will take over the coming months and years to interrupt and disable terror financing flows and intensify counter-terrorism activities globally."

The deal meant Qatar was "the first to respond" to Trump's call at a summit in Riyadh earlier this year "to stop the funding of terrorism", Tillerson said, suggesting such deals could be signed with the other Arab states as a step toward ending the crisis.

- 'Insufficient' step -

But Tuesday's initiative was brushed aside by the Arab countries that imposed sanctions on Qatar last month.

"This step is insufficient," said a joint statement published by Saudi state news agency SPA, adding that the four states would "carefully monitor the seriousness of Qatari authorities in combating all forms of financing, supporting and harbouring terrorism."

Commitments made by Qatari authorities "cannot be trusted," the statement added, citing previous agreements that have allegedly not been honoured.

The Saudi-led bloc has issued a list of 13 demands for Qatar including closing broadcast giant Al-Jazeera, downgrading ties to Iran and shutting a Turkish military base in Doha.

The four countries on June 5 announced sanctions, effective immediately, against Qatar over accusations Doha supported Islamist extremism and was too close to Iran.

They severed all diplomatic ties, suspended transport links with Doha and ordered all Qataris to return home within 14 days.

Qatar refused to comply with the ultimatum and has consistently denied accusations of ties to Islamist groups.

In a sign of the magnitude of the challenge facing Tillerson, a columnist in Saudi Arabia known for expressing views similar to those of the palace even cast doubt over the US diplomat's impartiality.

"The secretary of state can take Qatar's side if he wants to, but he has to realise that he will be further complicating an already complex matter and prolonging the crisis," Abdulrahman al-Rashed wrote in Saudi daily Arab News.

Tillerson's visit is the latest in a series by officials to the region, including UN diplomats and the foreign ministers of Germany and Britain, to try to resolve the row.

French Foreign Minister Jean-Yves Le Drian will visit the Gulf this weekend, with stops in Qatar, Saudi Arabia, the UAE and Kuwait.