STOCKHOLM (Reuters) - Estonian security police have detained two Estonian men, one of whom has been working at the Baltic country’s defense forces’ headquarters, on suspicion of spying for Russia, the prosecutor’s office said on Wednesday.

The office said they were suspected of passing Estonian and foreign classified information to the Russian Military Intelligence Service (GRU) for more than five years.

It said in a statement the men had been detained on Monday and a court had issued an arrest warrant to keep both men in custody the following day.

“We are talking about treason. We have grounds to suspect that state secrets were deliberately passed on to Russian Intelligence Service GRU,” Prosecutor Inna Ombler said in the statement.

Prime Minister Juri Ratas said in a separate statement the extent of the damage to Estonia had not yet been ascertained.

“The detention of two Estonian citizens on the suspicion of treason is a deeply disturbing incident,” he said.

Estonia, a NATO and European Union member since 2004, was invaded by the Soviet Union in 1940 and became independent in 1991.