Russia’s ambassador to the United Nations, Vitaly Churkin, has died.

The diplomat was due to celebrate his 65th birthday on Tuesday, but died while at work in New York on Monday, Russia’s foreign ministry said in a statement. The cause of the veteran diplomat’s death is unclear.

Churkin had served as ambassador to the UN since 2006, developing a reputation as a steadfast defender of Russian policy.

Russia’s foreign ministry paid tribute to an “outstanding diplomat”, while Farhan Haq, deputy spokesman for the UN secretary-general’s office, said he was “quite stunned”.

Samantha Power, the former US ambassador to the UN, took to social media to pay tribute to Churkin, writing on Twitter: “Devastated by passing of Russian UN Amb Vitaly Churkin. Diplomatic maestro and deeply caring man who did all he could to bridge US-RUS differences.”

Churkin had previously served as an envoy to Canada, Belgium and as a special representative to the talks on former Yugoslavia in the early 1990s.