MOSCOW (Reuters) - Gazprom CEO Alexei Miller was not on Russian President Vladimir Putin's state visit to China because he was holding talks with the Iranian leadership, the world's biggest natural gas company said on Thursday.

Gazprom, which holds about 16% of global gas reserves and employs hundreds of thousands of people, was once one of Russia's most powerful corporate empires - so powerful it was known as a state within the state.

But the loss of much of the European market due to Russia's war in Ukraine has hit it hard.

"Alexei Miller held talks with the Iranian leadership on the dates of Putin's visit to China," Gazprom said.

It was not immediately clear what else Miller, 62, a close ally of Putin who has run Gazprom since 2001, was talking to Iran about.

Gazprom said on Wednesday that Miller was on a working visit to Iran, a major producer of natural gas, where Miller met Iran's First Vice President Mohammad Mokhber and Iranian Oil Minister Javad Owji.

Gazprom's gas exports to Europe have fallen to post Soviet-lows amid political fallout from the war in Ukraine and after the major Nord Stream pipelines were damaged by mysterious blasts.

Gazprom plunged to a net loss of 629 billion roubles ($6.9 billion) in 2023, its first annual loss in more than 20 years, amid dwindling gas trade with Europe, once its main sales market.

Russia has been in talks for years about building the Power of Siberia-2 pipeline to carry 50 billion cubic metres of natural gas a year from the Yamal region in northern Russia to China via Mongolia.

Its capacity would be almost as much as the now idle Nord Stream 1 pipeline under the Baltic Sea that was damaged by explosions in 2022.

(Writing by Guy Faulconbridge; Editing by Andrew Osborn)