Former US ambassador to the UN Bill Richardson has died aged 75, his foundation has announced.

Serving under President Bill Clinton, he won admiration for his commitment to securing the release of US citizens detained around the world.

He continued that work out of politics, last year travelling to Moscow to discuss the release of detained basketball star Britney Griner.

Mr Clinton hailed Mr Richardson as a "masterful" negotiator.

"Bill worked tirelessly for the causes of freedom, fairness, and opportunity," Mr Clinton said in a statement on Saturday night.

"Whether in an official or unofficial capacity, he was a masterful and persistent negotiator who helped make our world more secure and won the release of many individuals held unjustly abroad."

Born in Pasadena, California, in 1947 to a Spanish-born mother and a Nicaraguan-born father, Mr Richardson grew up in Mexico City before attending boarding school in Massachusetts.

After graduating from Tufts University in 1970, he earned a master's degree in 1971. Soon after he embarked on a career in politics which would see him hold major national and state-wide offices.

In 1983 he was elected to the US House, representing New Mexico's Third District.

Over the next five decades in politics he developed a specialisation in diplomatic negotiations, skills which would see President Clinton tap him as his envoy to the UN in 1997.

The next year he became Mr Clinton's energy secretary, serving through to the end of the administration.

In 2002, he became the only Hispanic leader of a US state when he won the New Mexico governorship. His enduring popularity in the state saw him re-elected to a second term in 2006 by a record margin of 68% to 32%.

In his statement, Mr Clinton said his term as governor entrenched Mr Richardson's status as a "trailblazer", adding that his "career helped pave the path for other Latino Americans to serve at the highest levels of American government".

His success in office sparked a renewed interest in national politics, and in the run-up to the 2008 presidential election he launched a long-shot bid for the Democratic Party's nomination.

Despite playing a key role in Mr Clinton's cabinet, his public endorsement of Barack Obama after his withdrawal - instead of Hilary Clinton - was viewed as a betrayal by many Clinton supporters.

Mr Obama later nominated him as secretary of commerce, but he withdrew because of a pending investigation into allegations of improper business dealings - a probe that was later dropped.

Soon after leaving governorship in 2011 he launched his non-profit foundation, the Richardson Centre for Global Engagement, where he renewed his work seeking the release of detained Americans.

He was involved in efforts to release US basketball star Brittney Griner from a Russian prison in December after she was convicted of a drug offence.

He also met with Russian government officials in the months prior to the release of US Marine Trevor Reed in a prisoner swap.

And in 2021, he helped broker a deal for the release of American journalist Danny Fenster from a Myanmar prison.

Previously, he helped secure the release of US nationals detained in North Korea and he also held talks with Pyongyang diplomats in efforts to calm tensions between the two Koreas.

His work dealing with autocratic regimes once saw him jokingly refer to himself as the "the informal under secretary for thugs". But his work saw him nominated for the Nobel Peace prize several times.

The Richardson Centre hailed him as a "champion for those held unjustly abroad".

"He lived his entire life in the service of others - including both his time in government and his subsequent career helping to free people held hostage or wrongfully detained abroad," the foundation said in a statement.

"There was no person that Governor Richardson would not speak with if it held the promise of returning a person to freedom."

US President Joe Biden led tributes to Mr Richardson, who he called "a patriot and true original".

"Few have served our nation in as many capacities or with as much relentlessness, creativity, and good cheer," Mr Biden said. "His most lasting legacy will be the work Bill did to free Americans held in some of the most dangerous places on Earth."

"He will be deeply missed," Mr Biden added.

Democratic Senator Bob Menendez hailed Mr Richardson as "a quintessential public servant in every sense of the word".

"He was dedicated to improving the lives of those around him - whether it was the people of New Mexico as our nation's only Hispanic governor during his two terms or many Americans unjustly detained by despotic regimes around the world."

Reacting to news of the top diplomat's death, New Mexico Senator Martin Heinrich said: "Richardson's legacy will have a lasting impact."