TALLINN (Reuters) - Estonia has detained a man suspected of being a Russian agent operating against the Estonian state, the prosecutor s office said on Tuesday. The man was stopped at the weekend as he prepared to leave Estonia for Russia from the north-east border town of Narva. He was formally charged on Monday in a Tallinn court.    A person, who is a Russian citizen, is suspected of acting as an FSB agent in the preparation of a computer crime against the Republic of Estonia,  the prosecutors office said.  The target of his action against Estonia was the Estonian state authorities.  No further details were given. Russia s Federal Security Service, or FSB, is a successor to the KGB, the Soviet Union s security service. Neither the FSB nor the Russian Foreign Ministry immediately replied to requests for comment.  Estonia, occupied by the Soviet Union until 1991 but now a member of NATO and the European Union, has a strained relationship with its powerful neighbor.   In May this year, Estonia expelled two Russian diplomats posted at Moscow s consulate in Narva for contacts with local officials that were considered inappropriate for diplomats.   Earlier in the same month, an Estonian court handed out a five-year prison sentence to a Russian national for spying for the foreign intelligence unit of the Russian armed forces.    