Two Russian military aircraft were spotted near the island of Gotland off Sweden's Baltic coast Monday morning. Tensions have escalated between the two nations since June, following discussions by Sweden and neighboring Finland on joining the NATO military alliance.

Two Russian SU-27 fighter jets were observed by Swedish military aircraft during a routine sweep. The planes did not technically violate international airspace, said Jesper Tengroth, press secretary of the Swedish Armed Forces, adding the planes were "of interest" to the Swedish military, the Local reported.

Sweden, a nation that has often shied away from military alliances, has begun reconsidering its stance and has participated in NATO exercises in the region in recent months. In response to Sweden's potential membership in NATO, the Russian ambassador to Sweden responded with a thinly veiled threat in June.

"Russia will have to resort to a response of the military kind and reorientate our troops and missiles," said Ambassador Viktor Tatarinstev in an interview with a local Swedish newspaper. “The country that joins NATO needs to be aware of the risks it is exposing itself to.”

Russia has been building up its presence in the Arctic Circle and near the Baltic Sea bordering northern Europe in the past several months. Moscow submitted a bid to the United Nations in August staking its claim to more than 460,000 square miles of Arctic territory. The Russian military also announced plans to spend $93 million on a high-tech monitoring system in the Arctic to be installed by 2025.

Russian President Vladimir Putin had previously said his nation's heightened activity in the region was a response to NATO's increasing presence on Russian borders.  

Sweden in turn has upped its military expenditures, raising its 2016-20 military budget by $1.18 billion. “We are now seeing that Russia's military activity in the area is increasing, both in terms of scope and content. It is about controlling and boosting units' deployability," Sweden's Defense Minister Peter Hultqvist wrote in a Swedish newspaper Monday before the SU-27 incident, the Local reported.