Russian citizens Yury Anisimov and Alexander Smirnov have been placed under arrest by a Turkish court ahead of trial over the murder on November 1 last year of Chechen Abdulvakhid Edelgireyev in Istanbul, the Haberturk daily said.
It said they were first detained on April 8 in Istanbul while carrying out reconnaissance work for a new attack. It was not immediately clear which branch of the Russian security services they are suspected of belonging to.
"They have a lawyer, we are in contact with them and intend to follow how their rights and interests are being respected. They have no complaints about the conditions of their detention," he was quoted as saying by the RIA-Novosti news agency.
The Haberturk report said the two Russians had left Turkey in the wake of the Edelgireyev assassination but then returned on April 4 to plan a new action.
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They were said to have entered Turkey on fake passports, following an investigation lasting six months.
He was killed on November 1 by a bullet fired through the front window of his car close to his home in Istanbul.
Istanbul has become a known hub for exiles from Russia's troubled southern regions like Chechnya and Dagestan and many are believed to be on the run from the Russian security forces.
The arrest of the two alleged undercover agents comes as Turkey and Russia are experiencing their worst row for years following Ankara's shooting down of a Russian warplane on the Syrian border on November 24.