A US envoy tasked with resolving the diplomatic crisis between Qatar and its Gulf Arab neighbours has quit his post citing "unwillingness" of regional leaders to engage in dialogue, according to CBS News.

The US-based broadcaster said on Tuesday that Anthony Zinni, a retired Marine general, resigned after realising he "could not help resolve" the 18-month-long dispute, in which four Arab countries cut diplomatic and transport ties with Qatar, after accusing it of supporting "terrorism".

Qatar denies the charges.

The quartet - Saudi Arabia, the United Arab Emirates (UAE), Bahrain and Egypt - also imposed a land, sea and air blockade on the small Gulf state.

Zinni, a former commander of the US Central Command, said he was stepping down because of the "unwillingness of regional leaders to agree to a viable mediation effort that we offered to conduct or assist in implementing."

A spokesperson for the US State Department confirmed Zinni's resignation to The Associated Press news agency.

Robert Palladino said US President Donald Trump's administration will continue to pursue Zinni's work, which also included discussing with regional leaders the idea of a NATO-like group called the Middle East Strategic Alliance (MESA).

Observers say the murder of Saudi journalist Jamal Khashoggi inside the kingdom's consulate in the Turkish city of Istanbul upended plans for US-brokered talks to resolve the Gulf crisis in Washington in October.

Sigurd Neubauer, a Gulf political analyst, said that without an end to the Qatar blockade and no sign of the White House hosting a summit anytime soon, an alliance such as MESA has no chance of being stood up.

"Even though he [Zinni] was tasked to help resolve the crisis, once that was no longer attainable, at least in the current environment, he sought to repair the rift through integrated military cooperation," Neubauer told Al Jazeera.

"And with the murder of Khashoggi, that became an impossibility."

Failed mediation

Zinni was appointed by former Secretary of State Rex Tillerson in August 2017, two months after the dispute between the Saudi-led quarter and Qatar erupted.

His resignation coincided with a Middle East tour by Secretary of State Mike Pompeo that includes stops in the six countries that form the Gulf Cooperation Council (GCC), as well as Egypt and Jordan.

State Department officials have said Pompeo hopes his trip will fortify the GCC, which has been weakened by the Gulf crisis, and organise a summit of its leaders in the United States later this year. Saudi Arabia, the UAE, Bahrain and Qatar are all GCC members.

Mediation efforts by Kuwait have also failed to end the crisis so far.

The boycotting states insist Doha must meet a list of demands submitted to it at the start of the dispute, which include closing down the Al Jazeera Media Network, reducing ties with Iran and shutting a Turkish military base in Qatar.

The Qatari government said the demands amounted to an unlawful intervention against its sovereignty.

In November, Saudi Arabia and Egypt said the blockade will continue and they were not willing to make "any concessions" towards Doha.

Qatar's Emir Sheikh Tamim bin Hamad Al Thani has said that his country remained ready to hold talks to end the dispute.

"In the Gulf crisis our position remains unchanged - lifting the blockade and settling the differences via dialogue," he said in December.

Despite an initial disruption to its supply chains, Qatar has managed to weather the embargo by establishing new trade links, primarily with its ally Turkey, and injecting around $40bn from its ample foreign currency reserves into the economy.