Four Ukrainian soldiers were killed in a government-controlled town in the Donbass region of the country Sunday when their vehicle was hit by what police said was an artillery shell fired from territory held by pro-Russian rebels, Reuters reported. At the time of the blast, they were crossing a bridge outside the town of Schastye, about 105 miles northeast of the city of Donetsk.

A military representative said on Ukrainian television that two other soldiers were killed and one wounded in a separate incident near the settlement of Shyrokyne that was attributed to a land mine. Those deaths came shortly after a number of others related to land mines in the region, Reuters said. Meanwhile, an unconfirmed report claimed the explosion in Schastye also was caused by a land mine, as opposed to an artillery shell.

The Ukrainian government and the Russian-backed separatists agreed to a ceasefire deal in Minsk, Belarus, in February. The negotiators encompassed leaders of Ukraine, Russia, Germany and France. The ceasefire was largely ineffective in its first week, but has generally held in the wake of a rebel push to take key areas in Donetsk and Luhansk.

Both sides agreed to pull back heavy artillery from the ceasefire line, but sporadic shelling and small-arms fire has become the norm since the agreement was reached in Minsk. While most heavy weaponry has been pulled back, both sides have kept those guns at the edge of the ceasefire line, creating a tense standoff.

Artillery and rocket strikes have accounted for about 60 percent of all Ukrainian military casualties during the government’s yearlong campaign to wrestle back territory in the eastern reaches of the country. Both NATO and Ukraine have alleged Russia has sent heavy equipment and troops to help the pro-Russian separatists, offering satellite imagery as evidence of those actions. However, Russian President Vladimir Putin’s government has denied any role in the conflict.