"NATO stands in solidarity with our ally Estonia against any threat to their sovereignty," Stoltenberg said on the social media platform X after speaking with Estonian Prime Minister Kaja Kallas.

Russian border guards on Thursday staged a pre-dawn operation removing buoys from the Narva River marking the border and used by boats to avoid accidentally straying into foreign waters.

Estonia says that, since Russia launched its all-out invasion of Ukraine in February 2022, tensions have risen over that border area, with Moscow contesting the placement of the floating markers.

Kallas has said the buoys' removal appeared to be part of a "broader pattern" by Moscow to use "tools related to the border to create fear and anxiety".

The European Union's foreign policy chief Josep Borrell earlier Friday called Russia's removal of the buoys "unacceptable".

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