(CNN) Nicholas Burns, a respected, long-time Foreign Service officer who has served in both Republican and Democratic administrations, has joined former Vice President Joe Biden's foreign policy team as a formal adviser, CNN has learned.

The former ambassador was hired by longtime Biden foreign policy aide Anthony Blinken.

Burns -- who is the executive director of the Aspen Security Forum -- is highly critical of President Donald Trump and says he thinks Biden has the best chance of defeating the Republican incumbent.

Burns, 63, is a professor at Harvard at the Kennedy School of Government, and served as the Under Secretary of State for Political Affairs under George W. Bush's administration.

He served for five years -- from 1990-1995 -- on the National Security Council staff under Presidents George H.W. Bush and Bill Clinton. One position he had was a special assistant and senior director for Russia, Ukraine and Eurasia Affairs.

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