Anthony Zinni, a retired Marine Corps general and former head of U.S. Central Command who has been working as an envoy for the Trump administration to resolve a dispute with Qatar, has resigned from his position with the State Department. He is the latest four-star general to exit the administration.

Zinni resigned after realizing he could not help resolve the Qatar 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," he told CBS News. Zinni also felt there was no need for his involvement with the Middle East Strategic Alliance (MESA) concept — a NATO-style security alliance — that he was asked to introduce to regional leaders, since other members of the administration are carrying it forward.

A senior State Department official described Zinni's departure as a "soft resignation."

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Zinni had originally agreed to work as a special adviser to the secretary of state on Middle East issues at the request of then-Defense Secretary Jim Mattis and then-Secretary of State Rex Tillerson — both of whom have since left the administration in the wake of significant policy differences with Mr. Trump.

Zinni later coordinated with former national security adviser H.R. McMaster and Trump son-in-law and senior adviser Jared Kushner, then later current national security adviser John Bolton. Zinni described his working relationship with them as "excellent," and the inter-agency cooperation on the issues he worked on "impressive."

Zinni also served as former President George W. Bush's special envoy to the Middle East.

— CBS News' Russell Midori contributed to this report