MOSCOW: Russia’s FSB security service said on Thursday it had opened a criminal case against a journalist working for CNN who it said had illegally crossed the Russian border to film a report inside the Kursk region after a Ukrainian cross-border incursion.

The FSB named the journalist as Nick Paton Walsh, a British citizen who works as CNN’s chief international security correspondent. Moscow had banned him from entering Russian territory as part of its standoff with the West.

The FSB said it had opened similar criminal cases against two Ukrainian journalists, Diana Butsko and Olesya Borovik.

The security service, the main successor to the Soviet-era KGB, said in a statement that international arrest warrants for the three journalists would soon be issued. The maximum punishment for anyone found guilty of illegally crossing the Russian border is five years in jail, the FSB said.

CNN said in a statement that Paton Walsh’s reporting trip had been legitimate.

“Throughout this conflict our team has delivered factual, impartial reporting covering both the Ukrainian and Russian perspectives on the war,“ CNN said.

“Our team was invited by the Ukrainian government, along with other international journalists, and escorted by the Ukrainian military to view territory it had recently occupied. This is protected activity in accordance with the rights afforded to journalists under the Geneva Convention and international law,“ CNN said.

In the CNN broadcast, journalists travelled with a Ukrainian military convoy from Ukraine to the town of Sudzha, where they encountered a nearly deserted town with a few dozen elderly residents remaining.

Earlier this week, Russia summoned a senior U.S. diplomat in Moscow to protest what it called “provocative actions”of American journalists reporting from Kursk.

Russia’s Foreign Ministry said on Aug. 16 it had also summoned Italy’s ambassador over what it called the “illegal border crossing” into Kursk by a team of correspondents from Italian state broadcaster RAI. The FSB later opened criminal cases against two RAI journalists.

Ukraine’s lightning incursion into Kursk, the biggest into Russia by a foreign power since World War Two, began on Aug. 6 when thousands of Ukrainian troops crossed Russia’s western border.

Russia, which is still trying to expel Ukrainian forces from Kursk, said on Thursday its troops had beaten back an attempt by a Ukrainian force to infiltrate its border in Bryansk, a different region.