Sushchenko, Ukrinform's Paris correspondent, was detained on September 30
Russia's FSB security service on Monday said it had detained a Ukrainian journalist for spying, prompting Kiev to call for his immediate release.
Journalist Roman Sushchenko -- who has worked for Ukrainian state news agency Ukrinform since 2002 -- was detained in Moscow "while conducting espionage activities," Russia's FSB told Interfax news agency.
Russia's security service said that Sushchenko was working for Ukrainian military intelligence and "collecting state secrets... that could damage the defence capability of the state if leaked abroad."
Sushchenko, Ukrinform's Paris correspondent, was detained on September 30 upon arriving in Moscow on holiday, the Ukrainian agency said.
Russian lawyer Mark Feigin, who has defended a number of Ukrainian citizens in Russia in high-profile cases, told AFP that Sushchenko was being held at Moscow's Lefortovo prison.
Kiev has called for Sushchenko's immediate release and on Russian authorities to allow him to receive Ukrainian consular assistance.
"We demand that our consular officers be immediately provided with access to Roman," Ukrainian foreign ministry spokeswoman Mariana Betsa told AFP, stressing that minister Pavlo Klimkin was set to address Sushchenko's detention at a Council of Europe meeting on Monday.
Moscow's relations with Kiev are in shambles over Russia's 2014 annexation of Crimea and its support of a pro-Russian insurgency in eastern Ukraine that has claimed nearly 10,000 lives.
The conflict has also seen an unprecedented number of espionage and treason cases reach trial in Russia.
A Russian court last year sentenced an elderly Ukrainian citizen to six years in prison for allegedly attempting to smuggle sensitive missile defence technology out of the country.