James Warlick said Monday, November 21 he will step down as the U.S. Co-chair of the OSCE Minsk Group on December 31.

"It has been an honor to serve in the @StateDept for more than 30 years," he tweeted.

In a separate tweet, the American diplomat said he will be a Partner in Egorov Puginsky Afanasiev & Partners, or as he described "Russia's largest and most prestigious law firm."

Warlick was appointed the U.S. Co-chair of the Minsk Group in August 2013.Before that, he served as Deputy Special Representative for Afghanistan and Pakistan and lead negotiator for the Bilateral Security Agreement with Afghanistan. He served as Ambassador to Bulgaria from 2009-2012, Principal Deputy Assistant Secretary of State in the Bureau of International Organization Affairs from 2006 to 2009, and Director of the Office of European Security and Political Affairs from 2005 to 2006.