The Dutch intelligence service said on Thursday it had stopped a Russian spy posing as an intern from infiltrating the International Criminal Court, which is investigating war crimes in Ukraine.

The man, identified as Sergey Vladimirovich Cherkasov, 36, travelled to the Netherlands in April using a carefully constructed deep cover as a Brazilian citizen to take up his internship at the Hague-based tribunal.

But the Russian was unmasked as an agent of Moscow's GRU military intelligence service and refused entry to the Netherlands before being put on the next available flight back, the Dutch said.

Had he not been stopped, Cherkasov could have accessed "highly valuable" intelligence on the ICC's probe into war crimes in Ukraine or even influenced criminal proceedings, they added.

"The threat posed by this intelligence officer is deemed potentially very high," the Dutch AIVD, or General Intelligence and Security Service, said in a statement.

The head of the Dutch intelligence service, Erik Akerboom, said it was "very rare" to catch a Russian agent "of this calibre".

"The GRU has spent years creating this fake identity. It's an enormous effort," he was quoted as saying by the Dutch ANP news agency.

The Russian was a so-called "illegal" -- spy parlance for an agent who has lived abroad under a fake identity for years and is therefore "difficult to discover", the Dutch intelligence service said.

He travelled to the Netherlands under the name of a 33-year-old Brazilian citizen named Viktor Muller Ferreira, using a "well-constructed cover identity by which he concealed all his ties with Russia in general, and the GRU in particular".

But the Dutch pinpointed him as a "threat to national security" and notified the immigration service.

"On these grounds the intelligence officer was refused entry into the Netherlands in April and declared unacceptable. He was sent back to Brazil on the first flight out," the AIVD said.

The Russian's internship would have given him access to the ICC's building and systems at a time when it is probing war crimes in Ukraine, including alleged Russian crimes since the conflict broke out on February 24.

"For those reasons, covert access to International Criminal Court information would be highly valuable to the Russian intelligence services," the AIVD said.

Had the Russian spy succeeded "he would have been able to gather intelligence there and to look for (or recruit) sources, and arrange to have access to the ICC's digital systems," it added.

"He might also have been able to influence criminal proceedings of the ICC." (AFP)