ABC Europe correspondent Kathryn Diss and camera operator Fletcher Yeung have become the subject of a criminal investigation in Russia, where authorities claim they "illegally" crossed the border.

On August 31, Diss and Yeung were escorted by a Ukrainian military unit to Sudzha, a Russian town in the Kursk region, which is currently occupied by Ukraine.

While the ABC has been to Ukraine repeatedly since Vladimir Putin's full-scale invasion of the country in 2022, this was the first time a crew crossed the border into Russia.

The duo was there to report on Ukraine's decision to launch a surprise incursion into Russian territory in early August, advancing up to 30 kilometres into the region of Kursk.

During the brief visit, they spoke to Sudzha residents about life under Ukrainian occupation, and their hopes for the future.

On Friday, Russia's Federal Security Service (FSB) announced it had opened a criminal investigation into the pair, claiming they had "illegally crossed the Russian border into the Kursk region".

The agency said it had opened a total of 12 criminal cases against foreign journalists for the same "illegal acts".

In a statement, the ABC said: "We reject Russia's claim that the ABC's reporters have done anything illegal."

"They were reporting from occupied territory in a war zone and in full compliance with international law.

"Their reporting was done in the interests of keeping the public fully informed on a story of international importance. They are being fully supported by the ABC."

Multiple outlets have made trip

The ABC is not the only news outlet to visit Ukraine-occupied Russia in recent weeks, with German broadcaster Deutsche Welle, America's CNN and Italian public broadcaster RAI also making the journey.

According to Russian state media, the country's Federal Security Service (FSB) launched criminal investigations into a number of foreign journalists who it claims have illegally crossed the Russian border. 

Italian journalists Stefania Battistini and Simone Traini were the first foreign journalists to visit Sudzha last month. 

"Trying to put Italian journalists Stefania Battistini and Simone Traini on trial seems to be a desperate attempt by Russian authorities to intimidate and silence international journalists covering the Russian-Ukraine war," Gulnoza Said, from The Committee to Protect Journalists, said.

"Russian officials must stop their harassment of journalists and respect the essential role of the press in conflict zones."

Illegally crossing the border carries a penalty of up to five years in prison under the Russian Criminal Code.

Meanwhile, last year the International Criminal Court (ICC) issued an arrest warrant for Russian President Vladimir Putin, accusing him of being responsible for the war crime of illegal deportation of children from Ukraine.

And most countries are united in their stance that Russia's invasion of Ukraine violates Article 2(4) of the UN Charter.

This charter requires UN member states to refrain from the "use of force against the territorial integrity or political independence of any state".

Russia has banned hundreds of Australians 

Ukraine is currently occupying the region under the 1907 Hague Regulations.

"The authority of the legitimate power having in fact passed into the hands of the occupant, the latter shall take all the measures in his power to restore, and ensure, as far as possible, public order and safety, while respecting, unless absolutely prevented, the laws in force in the country," Article 42 states.

Russia is a party to the Hague Regulations 1907.

Diss and Yeung said they went to Sudzha because they believed it was in the public interest to understand this critical juncture of the war.

"Crossing the border here doesn't just carry the physical risk of being in a war zone; it also means that as individuals, we're unlikely to ever be able to return to Russia," they explained in their story.

Since the full-scale invasion of Ukraine began in February 2022, Russia has put more than 200 Australians on a list permanently banning them from entering the country.

Among them are several ABC journalists who have reported on the war, including Sarah Ferguson, Isabella Higgins, Emily Clark and Eric Campbell. 

The Russian foreign ministry said at the time they were targeted for the "spreading of false information about" the country.