Deputy Secretary of State John Sullivan will be nominated by President Trump to take over as ambassador to Russia following the announcement that former Utah Gov. Jon Huntsman will be leaving that post in October, an administration official told Fox News.

Sullivan, a lawyer with a long history of federal service, has been deputy secretary of state since May 2017 when the Senate confirmed him 94-6. He briefly led the State Department in the period between former Secretary of State Rex Tillerson’s dismissal in March 2018 and Mike Pompeo’s swearing-in the following month.

Sullivan’s expected nomination to one of the most important ambassador positions in the world comes just weeks after Huntsman sent a resignation letter to Trump and as relations between Washington and Moscow remain on edge.

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“American citizenship is a privilege and I believe the most basic responsibility in return is service to country,” Huntsman said in the letter to Trump. “To that end, I am honored by the trust you have placed in me as the United States ambassador to Russia during this historically difficult period in bilateral relations.”

The posting in Russia was Huntsman’s third as ambassador. He also served as ambassador to Singapore from 1992 to 1993 and ambassador to China from 2009 to 2011. In his letter, Huntsman urged Trump to take a firm stance with Russia, saying: “Going forward, we must continue to hold Russia accountable when its behavior threatens us and our allies."

Huntsman’s resignation takes effect Oct. 3.

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Sullivan’s career in government dates back to 1991, when he served as counselor to Assistant Attorney General J. Michael Luttig in the Justice Department’s Office of Legal Counsel. During the administration of President George W. Bush, Sullivan served as deputy general counsel to the State Department before moving on to the role of general counsel at the Department of Commerce.

Sullivan was also the chairman of the U.S.-Iraq Business Dialogue – an economic advisory forum – during the Obama administration.