According to Russian news agencies, Finland’s embassy in Moscow received a letter containing an unknown powder and reported the incident to Russian authorities.

Since Finland formally joined NATO on April 4, becoming the 31st member of the US-led military alliance, relations between Moscow and Helsinki have worsened dramatically. Finland and Russia share a long geographical boundary.

According to the RIA news agency, the embassy received three letters on Thursday, one of which contained a substance.

“In line with the security rules of the Finnish foreign ministry, the letters in question were handed to official representative organs of Russia which will study the matter,” RIA quoted the embassy as saying.

The embassy said it had also informed Russia’s foreign ministry of the incident.

Finland’s decision to join NATO ended seven decades of strategic non-alignment which began after the country repelled an attempted Soviet invasion during World War Two. In the postwar period it opted to maintain friendly ties with Moscow.

But Russia’s invasion of Ukraine in February 2022 prompted Finns to seek security under NATO’s collective defense pact, which states that an attack on one member is an attack on all.