Kuwait, which is mediating a peace deal, has secured a 48-hour extension to the deadline for Qatar to respond to the demands.
Saudi Arabia, the United Arab Emirates, Bahrain and Egypt severed diplomatic and transport links with Qatar last month on the ground it supports terrorism and meddles in their internal affairs.
Mr Roberts believes the issues surrounding Qatar and neighbouring gulf states could continue long after the 48-hour deadline.
He also revealed that Qatar's neighbors are plotting to topple its emir and warned the Saudi-led Arab states that, historically, Qatar had proven it was not an easy country to be "swallowed up".
He added that it was too soon to speak of future measures against Doha given its response on Monday to the list of demands issued by Saudi Arabia, Bahrain, the UAE and Egypt.
Erdogan: New Turkish military operation in Syria possible
But the United States regards the Kurdish militia as a reliable force to fight the Islamic State terrorists in Syria. Mattis said the us gave the fighters weapons they needed for the urban fighting they are facing in Raqqa.
But the UAE foreign minister declined to say what action the countries may take against Qatar.
Abdulrahman Al-Thani was to deliver the letter from the ruler of Qatar, Sheikh Tamim bin Hamad Al-Thani to the Emir of Kuwait, Sheikh Sabah Al-Ahmad Al-Jaber Al-Sabah, who has been the main mediator during this crisis.
DOHA: Qatar's foreign minister said Tuesday that demands put to Doha by Saudi Arabia and its allies in the Gulf diplomatic crisis were impossible to meet. Qatar claims its an attempt to curtail its sovereignty. "Qatar is prepared to face whatever consequences, but as, I have mentioned ... there is an global law that should not be violated and there is a border that should not be crossed".
Qatar, which denies funding extremism, will submit a formal response on Monday.
Saudi Arabia's King Salman Al-Saud will not attend the G20 summit due to the Gulf crisis, Saudi diplomatic sources said on Monday.
The statement did not give any details of the call.