The head of Georgia's rebel Abkhazia region, Sergei Bagapsh, died Sunday in a Moscow hospital following a lung operation, officials in the pro-Russian administration said.

In power since 2005, Bagapsh, 62, was re-elected as head of the separatist region by a crushing margin in 2009, slightly over a year after Russian forces defeated Georgia in a five-day war and proclaimed its independence.

"This was completely unexpected. It was very sudden," Abkhaz parliament speaker Nugzar Ashuba told Russian state television.

"The situation remains calm," the Abkhaz official added.

Culturally and religiously distinct from Georgians, Abkhaz separatists waged a fierce war in the 1990s after the break-up of the Soviet Union that killed thousands and caused some 250,000 people, mostly ethnic Georgians, to flee their homes.

The Black Sea region and South Ossetia, also declared independent by Russia following the August 2008 war, are still claimed by Georgia and not recognised by the West.

A source in the Black Sea region's administration said Bagapsh underwent surgery in Moscow on May 21 and died of complications.

Alexander Ankvab, his deputy, has been appointed as acting president, with new elections required by the region's constitution within three months.

"We are deeply shocked by the death of Sergei Bagapsh," the acting head of Abkhazia told the Interfax news agency.

"This is a great loss for all the Abkhaz people," the acting leader said.

The region's security council was expected to meet later Sunday to discuss the situation.

There was no immediate official reaction to the news from Georgia, whose Western-backed President Mikheil Saakashvili is now facing almost daily street protests from pro-Russian opposition forces.

The acting leader Ankvab, 58, was born in Abkhazia and educated in Russia, according to his official biography.

He briefly served as interior minister after the Soviet Union's collapse before going into business and moving to Moscow for six years.

Ankvab served as prime minister between 2005 and 2010, and was appointed vice president in February 2010.