MOSCOW (Reuters) - Three teenage schoolboys were killed in eastern Ukraine when they triggered a mine in an area that has seen heavy fighting between separatists and Ukrainian troops, the region’s separatist authorities said on Sunday.

The boys all studied at the same school in Horlivka, a town controlled by separatist forces, and were aged between 13 and 15, Horlivka mayor Ivan Prikhodko wrote in a social media post.

A fourth schoolboy, who is 10, was injured in the explosion and is being treated in hospital for shrapnel injuries, Prikhodko wrote.

Denis Pushilin, leader of the separatist Donetsk region that includes Horlivka, issued a statement confirming the deaths.

Fighting broke out in 2014 between separatist forces in eastern Ukraine backed by Moscow and forces loyal to the pro-Western Ukrainian government in Kiev. Despite an internationally-brokered ceasefire, there are frequent flare-ups of fighting, with resulting loss of life.