Mekhti Logunov was arrested in August last year in Kharkiv, a city in northeast Ukraine near the front with pro-Russian separatists, according to the regional prosecutor.

"He was gathering information belonging to scientific institutions that were state secrets," notably in the military domain, the prosecutor said in a statement.

Logunov's trial took place behind closed doors because of the state secrets involved, his lawyer, Dmytro Tykhonenkov, told AFP.

Tykhonenkov said his client had been given a 12-year prison sentence but added he planned to appeal.

The scientist had previously publicly expressed his admiration for the separatist cause in eastern Ukraine.

"If I were younger, I probably would have joined (Igor) Strelkov," he said in an interview with a Ukrainian television channel, referring to the former rebel leader.

According to the Ukrainian television channel 1+1, the scientist obtained a secret report on Kiev's development of a new military technology and was preparing to take it personally to Russia before he was arrested.

The conflict between pro-Moscow separatists and the Ukrainian army has killed more than 10,000 people since it broke out in 2014.

Kiev and the West say Russia is giving military support to the rebels -- a charge Moscow denies despite many reliable reports of its troops being present in the conflict zone.