WASHINGTON (Reuters) - Anthony Zinni, a retired U.S. general who sought to resolve a dispute between Qatar and some of its neighbors and to forge an Arab security alliance to contain Iranian power, has resigned, CBS News reported on Tuesday.

CBS News quoted Zinni as saying he stepped down from his Trump administration assignment after realizing he could not resolve the dispute “because of the unwillingness of the regional leaders to agree to a viable mediation effort that we offered to conduct or assist in implementing.”

The diplomatic dispute pits Qatar against four Arab nations, Saudi Arabia, the United Arab Emirates, Bahrain and Egypt, which cut diplomatic and transport ties with Doha in 2017, accusing it of backing terrorism. Qatar denies supporting terrorism and there has been no sign of a resolution to the spat.

Zinni was also the lead negotiator for the creation of the proposed Middle East Strategic Alliance, which would bind Sunni Muslim governments in Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, Oman, Bahrain, Egypt and Jordan in a U.S.-led security, political and economic pact to counter Shi’ite Iran.

Asked about the report of Zinni’s resignation, the State Department thanked Zinni for his service and confirmed it would take the lead on Middle East Strategic Alliance, though it said he would remain available to consult if need be.

“General Zinni’s mission was to help introduce the concept of the Middle East Strategic Alliance and start a conversation with leaders in the region. That is happening and well underway thanks to his efforts,” said State Department spokesman Robert Palladino. “The Administration, through the interagency process led by the Department of State, will carry the mission forward.”

Zinni is the latest official to leave the administration, joining Jim Mattis, the former defense Secretary; John Kelly, the former White House chief of staff; and Brett McGurk, the former envoy for the global coalition to defeat Islamic State.