This follows May's incident when Russian authorities took it upon themselves to simply remove nearly half of the river buoys set up to mark the border.

Erik Liiva, head of the border and migration supervision department at the Police and Border Guard Board's (PPA) East Prefecture, said: "Almost all illegal crossings of the temporary control line this year have taken place in areas where the Russian side unilaterally removed the light buoys in May.

"These incidents are practically a daily occurrence, especially at weekends," Liiva added.

The PPA says the violations are generally unintentional navigation errors, but the issue is still taken seriously.

Fishermen who cross the line, even accidentally, will be apprehended – and potentially face a fine.

Liiva said the goal is "not to penalize anyone" and as much as anything is to serve as a warning to people on the Estonian side of the border about – given that the reception anyone accidentally crossing the borderline in the opposite direction is not likely to get as pleasant a reception on the totalitarian side of the frontier.

"The state may not be able to ensure their quick and safe return," Liiva said.

Navigation buoys mark the shipping route along the border as it lies in the lower reaches of the Narva River. The border here runs around halfway across the river, which is around 100 meters wide.

As noted the buoys also serve to warn fishermen about the proximity of the temporary control line.

This makes unintentional infringements of the border line primarily a concern for smaller vessels; larger riverboats are equipped with proper navigation systems, "Aktuaalne kaamera" reported.

As if to underscore the importance of the markers, in the stretch covered by the 26 remaining marker buoys, there have been no illegal border crossings, unintentional or otherwise, this year so far.

In any case, the PPA recommends using navigation devices when traversing the Narva River, even if this is simply a smartphone with GPS – though these are far from failsafe.

One leisure sailor, Anatoli Ivanov, head of the yacht club downstream at Narva-Jõesuu, told "Aktuaalne kaamera" that once his GPS signal indicated he was still in territorial Estonian waters, while his charts indicated he had already crossed over to the Russian side.

"Perhaps the Russian military is testing some equipment, and the signal gets lost," he speculated.

Twenty-five floating markers were removed by the Russian border guard at around 3 a.m. on the morning of Thursday, May 23, the PPA said at the time.

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