A Saudi-led bloc of Arab states hostile to Qatar on Friday took aim at Kuwaiti mediation and maintained a tough line even after US President Donald Trump offered to help resolve the crisis.

Saudi Arabia as well as the United Arab Emirates, Egypt and Bahrain cut ties with Qatar in June, accusing it of bankrolling Islamist extremist groups and of being too close to Iran.

They also shut down air, maritime and land links and imposed economic sanctions on Qatar. The gas-rich emirate denies the claims and accuses the four countries of an attack on its sovereignty.

In Washington on Thursday, Kuwaiti Emir Sheikh Sabah al-Ahmad Al-Sabah, a key figure involved in trying to mediate an end to the dispute, met with Trump and gave an upbeat assessment of his efforts so far.

At a joint news conference in Washington with the emir, Trump offered his own mediation in the three-month crisis, the worst to have gripped the Gulf region in decades.

"I would be willing to be the mediator," Trump told reporters.

"I would be willing to do so, and I think you'd have a deal worked out very quickly," he said. "I think it's something that's going to get solved fairly easily."

But in a statement early Friday, the Saudi-led bloc showed no signs of backing down as it questioned the Kuwaiti emir's statement that Qatar would be willing to accept their 13 demands.

"Dialogue on the implementation of the demands should not be preceded by any conditions," they said in the joint statement carried by the state-run Saudi Press Agency.

The demands include shutting Doha-based broadcaster Al-Jazeera, closing a Turkish military base in the emirate and downgrading Qatari diplomatic ties with Iran.

- 'Lack of seriousness' -

The bloc also voiced "regret" about the Kuwaiti ruler's statement "on the success of mediation in stopping military intervention".

Instead, the four Arab states stressed in their joint statement that "the military option has not been and will not be considered" under any circumstances.

They also criticised Qatari Foreign Minister Sheikh Mohammed bin Abdulrahman Al-Thani for comments he made after the meeting between Trump and the Kuwaiti emir.

In remarks aired on the Al-Jazeera television network, Sheikh Mohammed rejected the Saudi-led bloc's 13 demands, saying they "attacked the sovereignty" of his country.

"Setting preconditions for dialogue confirms Qatar's lack of seriousness in dialogue, combating and financing terrorism and interfering in the internal affairs of countries," the bloc said in its statement.

The United States has given mixed signals on its policy to the Gulf crisis while Kuwait has emerged as a key mediator.

Trump immediately expressed staunch support for Saudi Arabia after the Arab states announced sanctions against Qatar, but some other US officials including Secretary of State Rex Tillerson adopted a more measured tone.

Riyadh and Doha are both key allies of the United States.

Trump chose Saudi Arabia for his first overseas visit as president in May, two weeks before the Gulf crisis erupted.

Qatar is meanwhile home to a huge US air base, where the headquarters of Centcom -- the regional command which leads operations against the Islamic State jihadist group -- is based.

Last month Qatari and US paratroopers held a joint training exercise which American officials said reinforced "the enduring military-to-military" partnership between the two countries.