Turkey editor-in-chief of Russia's Sputnik agency detained in Istanbul
"Sputnik Turkey's editor-in-chief Mahir Boztepe has been detained. He is being taken to the Istanbul police headquarters," the website said.
Margarita Simonyan, editor-in-chief of Rossiya Segodnya which owns Sputnik, confirmed the detention on Twitter, and criticised the earlier detention of three other journalists in Ankara.
"Three others were at the police station in Ankara since yesterday where last night nationalists entered their apartments. Turkey, what's going on???" Simonyan tweeted.
She added that Ankara police had said the journalists were not in custody.
"Yet at the same time, we haven't had any contact with the same three since last night - when they went to the same police station to complain about the nationalists who entered their homes."
Sputnik said that the three journalists had been taken to an Ankara court for questioning and that Turkish police were searching the Russian news website's Istanbul office.
Sputnik also said a meeting was underway in Istanbul between an agency representative, their lawyer, officials from the Russian consulate and Turkish police.
The Turkish branch of Reporters Without Borders (RSF) tweeted on Sunday that police took statements from the three journalists in Ankara after Sputnik published an article in English titled: "The 'Stolen Province': Why Turkey Was Given A Corner Of Syria By France 80 Years Ago."
Colonial power France ceded the southern region to Turkey in 1938.
The Russian foreign ministry earlier criticised the detention of the three journalists.
The ministry said that "protesters" entered the journalists' homes and threatened them.
"We call on the Turkish authorities to intervene and ensure the safety of Russian media journalists," it said in a statement.
The relationship between Russia and Turkey has been strained since attacks blamed on the Russian-backed Syrian regime killed dozens of Turkish soldiers last month in the northwestern rebel-held Syrian province of Idlib.
While Ankara backs Syrian opposition fighters, Russia is a close ally of Damascus and has provided air power during the recent assault on Idlib.
Ties between the two countries deteriorated after Turkey shot down a Russian plane in 2015, but a rapprochement in 2016 led to closer links in multiple areas including energy and in Syria.
The arrest on Sunday came as Turkish forces "targeted" two Syrian regime planes over the embattled rebel bastion of Idlib in northwest Syria, Syrian state media said on Sunday, just moments after Turkey announced Operation Spring Shield.
The planes were "downed", according to a rebel group and the Syrian Observatory for Human Rights, a Britain-based war monitor.
The latest escalation shortly after Turkey said it was continuing a military operation against the Russian-backed Syrian regime in Syria's northwestern Idlib region.