Russia voiced regret Monday over Turkey's decision to convert the iconic Hagia Sophia in Istanbul back into a mosque.

Turkey's move last week followed a court ruling that revoked the landmark's museum status, and has triggered a global outcry.

The Russian Orthodox Church had already expressed dismay at the decision.

In a comment on the foreign ministry's website, spokeswoman Maria Zakharova said Moscow "noted with regret" Ankara's decision regarding the UNESCO World Heritage site.

Initially a Byzantine cathedral, it had already been used as a mosque before becoming a museum, which conferred a religious detachment that has now been severed. 

On Friday, Russian Orthodox spokesman Vladimir Legoida said "the concern of millions of Christians were not heard" by Turkish authorities.

Zakharova's comment came after deputy foreign minister Sergei Vershinin told reporters that Moscow viewed the move as "an internal Turkish affair in which neither us nor others should interfere." 

The Hagia Sophia was constructed as a cathedral during the Byzantine empire but converted into a mosque after the Ottoman conquest of Constantinople in 1453.

It is to reopen this month for Muslim worship as a mosque after almost nine decades as a museum.

Zakharova said that as a museum the "sacred place for all Christianity" had become "a symbol of peace and inter-faith unity."

Moscow is counting on Hagia Sophia's management to "fully comply with the status of a UNESCO site," she said.

"We expect any decisions on this unique landmark to take into account its unique significance for believers around the world."