An Estonian Army officer and his father have been arrested on suspicion of treason and spying for Russia, authorities say.

Estonia’s State Prosecutor's Office and the Internal Security Service said in a joint statement on September 5 that Deniss Metsavas and Pjotr Volin were arrested two days earlier.

Metsavas, 38, and his 65-year-old father are suspected of passing Estonian state secrets and other classified information to the Russian military intelligence service, also known as the GRU, since 2013.

Metsavas has served with the armed forces since 1998 in various positions including a mission with NATO troops in Afghanistan.

He became an army major in 2015.

Volin served in the KGB Border Troops during the Soviet-era.

According local media, the two men received Estonian citizenship in the 1990s. "It is clear that it was Metsavas who had access to classified information in the first place but the forwarding of classified information and bringing it to the GRU was committed by both individuals," prosecutor Inna Ombler told reporters.

The commander of the Estonian armed forces, General Riho Terras, said Metsavas was a “traitor" who caused significant damage to the small Baltic nation and its NATO allies.

"Damage has been done, a wound has been inflicted on the defense capability of Estonia," he said.

Occupied by the Soviet Union during World War II, Estonia gained independence in the Soviet collapse and is now a member of NATO and the European Union.

The former Soviet republic has witnessed several high-profile spy cases involving Russia since its independence in 1991.

In February, the two neighbors swapped two convicted spies after both men received presidential pardons, Estonian authorities said.

With reporting by AP, Delfi, and Baltija.eu