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.

Burns also was State Department spokesman for several years during the Clinton administration and was US ambassador to Greece.

Under George W. Bush, he was US ambassador to NATO and later the third-highest ranking State Department official – the Under Secretary of State – before retiring from public service in 2008.

Burns announced earlier this month that he was supporting Biden’s presidential bid.