Poland will build six watchtowers to survey its 200-kilometre-long border with the Russian enclave of Kaliningrad, the police said Monday.

The six towers will be up to 50 metres (164 feet) high and ready in June for round-the-clock surveillance, the spokeswoman for Poland's border police told the PAP news agency.

They will cost more than 14 million zloty (3.7 million euros, $3.8 million), Miroslawa Aleksandrowicz said, adding that 75 percent of the amount would come from an EU fund for external borders.

Kaliningrad is near the Baltic Sea, sandwiched between Poland and Lithuania, both EU members. Lithuania's President Dalia Grybauskaite said last month that Russia had sent nuclear-capable Iskander missiles to Kaliningrad, which could "reach even Berlin".

Russia's seizure and annexation of Crimea, support for separatists in eastern Ukraine and stepped-up military drills have caused unease in the Baltic states and Poland, which lay behind the Iron Curtain a quarter of a century ago.

More than three million Russians and an equal number of Poles passed through border posts to heavily militarised Kaliningrad last year.