The Ukrainian Foreign Ministry has protested the decision made by a Moscow court to prolong the arrest of journalist Roman Sushchenko, a citizen of Ukraine, who is charged with espionage in Russia.

"The prolongation of Roman Sushchenko's arrest in Russia on the basis of falsified charges is ungrounded. We protest, we demand his release," Ukrainian Foreign Ministry spokeswoman Mariana Betsa wrote on Twitter on Monday.

Later, the press service of the Ukrainian Foreign Ministry reported that the Russian court's refusal to change the measure of restraint for Sushchenko "is a sign of pressure on him and bias of the Russian judicial authorities."

"The closed regime of the court session, the court's rejection of the petition to admit the consul to participate in the hearing as a public defender, as envisaged by the Russian Criminal Procedure Code, confirm the purposeful political nature of the case against Roman Sushchenko," the ministry said.

"We call on the world community, international human rights institutions and the OSCE Representative on Freedom of the Media to take all possible legal and political measures to release and return to Ukraine a citizen of Ukraine, journalist Roman Sushchenko," reads the statement.

Earlier that day, the Moscow City Court left Ukrainian citizen Roman Sushchenko, who is charged with espionage in Russia, under arrest until January 13, 2018.

"The court has granted the investigative bodies' request to prolong the detention of accused Sushchenko until January 13, 2018," the court said on Monday.

Sushchenko is charged with espionage. He says he is not guilty.

The preliminary investigation into the case has now been completed. Attorneys are reading the case materials.