A Yemeni individual and an organisation that appeared on a US and Gulf list of terror financiers were previously highlighted by the quartet as backed by Qatar.
Now Qatar, which is under a boycott from Saudi Arabia, the UAE, Bahrain and Egypt over its links to extremist groups, has supported the sanctioning of those entities along with 11 more in a list published on Wednesday.
The sanctions were coordinated by the recently established Terrorist Financing Targeting Center, which includes the GCC countries and the US. The sanctions mostly target leaders and financiers of ISIL in Yemen and Al Qaeda in the Arabian Peninsula.
In June, the quartet issued a list of 59 people and 12 groups they said were linked to Qatar over terrorism. A further 18 individuals and entities were added to the list in July.
Qatar denied the allegations saying: “all individuals with links to terrorism in Qatar have been prosecuted”.
Some of those names were included in the list announced by US Treasury Secretary Steven Mnuchin in Riyadh on Tuesday, which was agreed to with all six GCC countries, including Qatar.
They include Al Rahma Charity Foundation and Abdulwahab Mohammed Abdulwahab Al Hamiqani, a fundraiser and recruiter for AQAP, who has orchestrated several terror attacks, the Treasury Department says.
The move is the first time the GCC has worked together since the Qatar crisis started on June 5 with the four Arab countries severing travel, trade and diplomatic ties to Doha.
However, the coordination on terror could be an attempt by Washington to ease tension between the countries.
“Given that one of the quartet's primary complaints vis-a-vis Qatar relates to allegations of support to militant groups, this type of joint effort could be part of US efforts to bring the parties together,” said Miriam Eps, regional security analyst at Lebeck International.
Ms Eps said it could also be a message that, despite the ongoing rift and reports of a possible postponement of the annual GCC summit, the continuation of the organisation is not at risk.
Many were looking to the GCC summit, which is set to take place in December, as an opportunity for Kuwait to bring the two sides together, with Emir Sheikh Sabah the chief mediator in the dispute.
“Of course, with the parties seeming unable to agree to even talks, the involvement of Qatar in these sanctions could simply be notable but not representative of any change in the current status quo,” Ms Eps said.
While designations of individuals and entities linked to militant groups are not uncommon for the individual countries, the latest sanctions list is unique due to the number of countries involved and the role played by the Terrorist Financing Targeting Center, which was established during Donald Trump's visit to Saudi Arabia in May.
“Technocratic cooperation on issues of shared concern can function as the nuts and bolts that can begin to repair the fractured relationships in the region, and rebuild some of the ties of trust and confidence that have been so damaged in recent months,” Kristian Ulrichsen, fellow for the Middle East at Rice University’s Baker Institute for Public Policy, said.