"We have informed the Russian delegation that this (NATO) accreditation is no longer current, and that therefore the basis for carrying out their work (as diplomats in Belgium) is no longer in place," she said.

"We have also asked them when they are leaving their posts."

NATO expelled the pair, including the son of Russia's ambassador to the European Union, last week in connection with an Estonian spy scandal which has already been through the courts.

A source close to the affair said that Belgium had not set a deadline for them to leave the country, but that once they were taken off the diplomat list, they would be expected to quickly head back to Russia.

"Unless they became illegals," the source added.

NATO's expulsion of the two Russian diplomats was part of a long-running investigation into an Estonian spy scandal case.

Former Estonian defence ministry official Herman Simm was arrested last September in a case which has proved deeply embarrassing for the Baltics NATO member amid concern that alliance secrets might have been leaked to Russia.

Simm pleaded guilty to treason and was sentenced in February to 12 years and six months in prison.

In a statement, Russia's embassy in Brussels warned NATO nations to carefully weigh their actions.

"We urge all NATO members to think of the consequences of what is happening. Of course, we will draw our own conclusions from this provocation," it said.

Tensions between Russia and its old Cold War enemy have risen since Moscow sent troops into Georgia in August last year, resulting in a long freeze in high-level ties between them.

AFP/Expatica