Russia has finished building a high-tech security fence along annexed Crimea's border with mainland Ukraine.
The fence, more than 60km (37 miles) long, is topped with barbed wire and has hundreds of sensors.
Russian forces annexed Ukraine's Crimea peninsula in March 2014 - a move condemned internationally. Crimea has a Russian-speaking majority.
Russia's FSB security agency says the fence is necessary to prevent "infiltration attempts by saboteurs".
An FSB statement, quoted by Russia's RIA Novosti news agency, said the fence would also thwart smugglers trading in illegal weapons, drugs, alcohol and other contraband.
The fence spans the neck of land connecting Crimea with Ukraine's Kherson region.
Most of its sensors pick up vibrations from any potential intruders, the FSB said, but some are also radio-location devices. Russia has similar equipment along its northern and eastern borders.
On Wednesday, Ukrainian President Petro Poroshenko lifted martial law in the country's border regions, which was imposed last month after Russia seized three Ukrainian naval vessels in the Sea of Azov.
On Friday, German Chancellor Angela Merkel and French President Emmanuel Macron demanded that Russia release 24 Ukrainian sailors it captured along with the two small gunboats and a tugboat last month in the Kerch Strait, between the Black Sea and the Sea of Azov.
The two leaders also asked for free access for all ships through the strait, in a joint statement.
Moscow accuses the 24 navy personnel of illegally crossing the Russian border, but Ukraine says Russia captured the boats illegally and accuses Moscow of military aggression.
Ukraine has also erected border fences since Russia's 2014 annexation of Crimea and the pro-Russian insurgency in eastern Ukraine. New fences went up in the Chernihiv and Kharkiv regions, north of Kiev - regions bordering on Russia. Ukraine also has a new fence in Kherson region, near Crimea.