Russia's FSB security service said 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 following 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.

It said the FSB had also opened similar cases against two Ukrainian journalists.

CNN, which on Aug. 16 broadcast a report from Sudzha, a Russian border town currently under Kyiv's control, did not immediately reply to a request for comment.

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

The FSB said in a statement that Moscow would soon issue an international arrest warrant related to the three journalists' cases. The maximum punishment for anyone found guilty of illegally crossing the border is five years in jail, it said.

Russia summoned a senior U.S. diplomat in Moscow earlier this week to protest over what it called the "provocative actions" of American journalists reporting from the Kursk region.

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

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