KYIV: The United Nations said Wednesday that around 13,500 square kilometers (5,000 square miles) of Ukraine’s lakes, rivers and coastlines are potentially contaminated with mines and explosives after the nearly four-year Russian invasion.

Even as the war is mostly fought on the ground, both sides have mined large areas near coastlines, and some undetonated projectiles from Russia’s daily aerial barrages end up in bodies of water.

“An estimated 13,500 square kilometers of Ukraine’s aquatic areas — including the Dnipro River, lakes, and Black Sea coastlines — are potentially contaminated with explosive remnants of war,” the United Nations Development Programme (UNDP) in Ukraine said in a statement.

Only 1.4 percent of the contaminated waters — roughly equivalent to the size of Puerto Rico — has been de-mined, with the removal of around 2,800 explosive devices.

Ukraine uses underwater robots in its de-mining efforts, and the UN said it had trained 15 specialist instructors as part of its support efforts.