The United States said Friday it was "disappointed" by Turkey's decision to turn the Byzantine-era monument Hagia Sophia back into a mosque and urged equal access for all visitors.

"We are disappointed by the decision by the government of Turkey to change the status of the Hagia Sophia," State Department spokeswoman Morgan Ortagus said.

"We understand the Turkish government remains committed to maintaining access to the Hagia Sophia for all visitors, and look forward to hearing its plans for continued stewardship of the Hagia Sophia to ensure it remains accessible without impediment for all," she said.

Turkish President Recep Tayyip Erdogan, who has chipped away at the Muslim-majority country's secularism, announced Muslim prayers on July 24 at the UNESCO World Heritage site.

A magnet for tourists worldwide, the Hagia Sophia was first constructed as a cathedral in the Christian Byzantine Empire but was converted into a mosque after the Ottoman conquest of Constantinople in 1453. 

Erdogan's announcement came after the cancellation of a decision under modern Turkey's secularizing founder Mustafa Kemal Ataturk to preserve the church-turned-mosque as a museum.

Erdogan went ahead despite an open appeal to the NATO ally by US Secretary of State Mike Pompeo, an evangelical Christian who frequently speaks about religious freedom.

In a statement last week, Pompeo called the museum status an "exemplar" of Turkey's "commitment to respect the faith traditions and diverse history" of the country and said a change risked "diminishing the legacy of this remarkable building."