“Recently, the opening of the Montreux Convention to debate within the scope of the authority to withdraw from international treaties and the Canal Istanbul project is a cause for concern,” the letter read.

The Ankara chief public prosecutor’s office said arrest warrants had been issued for the 10 and ordered four other suspects to report to Ankara police within three days, opting not to detain them because of their age.

They are accused of “using force and violence to get rid of the constitutional order”, NTV broadcaster reported.

The prosecutor’s office opened a probe on Sunday into the retired admirals on suspicion of an “agreement to commit a crime against the state’s security and constitutional order”.

The wording is similar to that prosecutors have used against other Erdogan critics who have been jailed in a crackdown that followed a failed coup in 2016.

The retired admirals detained on Monday included some of Turkey’s most famous naval commanders.

They included Cem Gurdeniz, often described as the father of Turkey’s controversial new maritime doctrine known as “Blue Homeland”.

The doctrine has grown in prominence, especially during tensions last year between Greece and Turkey over Ankara’s gas exploration in the eastern Mediterranean. It argues Turkey has rights to substantial maritime borders including the water surrounding some Greek islands, much to Athens’ chagrin.

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Also detained was Alaettin Sevim, who had previously been arrested in 2011 as part of the so-called Ergenekon alleged coup plot against the government, which was later discredited.

Before the arrest warrants were issued, state news agency Anadolu said the prosecutor’s office had determined who the “leaders” were of the open letter.

Turkish officials have reacted angrily to the letter, claiming it appears to be a call for a coup.

“Stating one’s thoughts is one thing, preparing a declaration evoking a coup is another,” parliament speaker Mustafa Sentop said on Sunday.

Coups are a sensitive subject in Turkey since the military, which has long seen itself as the guarantor of the country’s secular constitution, staged three coups between 1960 and 1980.

The attempt to overthrow Erdogan in 2016 was blamed on followers of US-based Muslim preacher Fethullah Gulen in the military.

“This is upsetting in the name of democracy,” Turkish Justice Minister Abdulhamit Gul said on Monday. “Our struggle against this dark mentality continues. The necessary response will be given within a legal framework.”

Hundreds protest against Turkey’s withdrawal from landmark women’s treaty

Erdogan’s proposed 75-billion-lira (US$9.8-billion) alternative to the Bosphorus and Dardanelles raises questions on whether existing agreements would apply to the new route.

Last month, parliament speaker Sentop said the president “could” withdraw from the convention. “He has the power,” he told HaberTurk channel on March 24. “But there is a difference between possible and probable.”

Erdogan assumed the power to pull Turkey out of treaties without parliament’s approval in 2018.

Last month he withdrew Turkey from the world’s first binding treaty to prevent and combat violence against women.