Russia's secret service agency, the FSB, said it has opened a criminal case against CNN journalist Nick Paton Walsh and two Ukrainian correspondents saying they illegally crossed into the country to film reports on Ukraine's incursion into Russian territory. The journalists -- which include Paton Walsh, a British national, and Ukrainian journalists Diana Butsko and Olesya Borovik -- reported from the Sudzha area of the Kursk region, where Ukraine has surprisingly taken a foothold inside of Russia in recent weeks. The incursion is the biggest into Russia by a foreign power since World War II.

"In the near future, they will be put on the international wanted list," the FSB said in a statement on August 22, adding all three face up to five years in prison. The journalists entered Russia embedded with a Ukrainian military convoy and travelled to the town of Sudzha in a reporting mission CNN said was a legitimate reporting trip. "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," CNN said in a statement.

"This is protected activity in accordance with the rights afforded to journalists under the Geneva Convention and international law," it added. The FSB move comes on the heels of a similar case launched last weekend against two Italian journalists who reported on Ukraine's offensive in the Kursk region. Simone Traini and Stefania Battistini were also accus.