Poland has ordered the expulsion of nine Russian diplomats accused of spying.

Russia said it would take "appropriate response measures" to what it called a "provocative act".

The Russian ambassador in Warsaw, Sergei Razov, was called to the Polish Foreign Ministry and told that nine of his country's diplomats were "persona non grata" in Poland.

A Polish Foreign Ministry source confirmed the nine would have to leave Poland and said their departure was expected within a week.

Polish Government spokesman Krzysztof Luft said the counter-intelligence agency "investigated and provided evidence of active espionage activities against Poland's interest in 1999 by a group of Russian diplomats".

Konstanty Miodowicz, a former Polish counter-intelligence chief who sits on the parliament's secret service committee, was quoted by the PAP news agency as saying the Russians used "aggressive operational resources" to gather classified information illegally.

'Unfriendly act'

Russia said it would respond to Poland's decision to expel nine Russian diplomats on spying charges, and saw it as an "unprecedented step" which soured bilateral relations.

"The Russian side cannot leave unnoticed such explicitly unfriendly and provocative act," a Foreign Ministry statement said.

"The Polish Government should realise that appropriate response measures become inevitable," the statement said.

Poland, a former member of the Soviet-led Warsaw Pact, joined the Nato military alliance last March along with Hungary and the Czech Republic. Russia staunchly opposed the move.

No senior Russian officials have visited Poland for several years.

Last year, Polish counter-intelligence arrested three army officers on charges of spying for Russia in the early 1990s.

Poland last expelled a Russian diplomat for spying in 1993.