Between 9 and 10 this morning, a salvo of unguided Grad rockets struck the eastern district of Mariupol, a government controlled city in eastern Ukraine. Two other rocket attacks struck in the area later in the day. As of this writing, local authorities are saying that 27 people were killed and 90 people injured. There haven’t been hostilities in Mariupol for months, but today’s attack was one of the deadliest in terms of civilian casualties since the war broke out in April 2014. It happened in the part of Mariupol closest to the frontline between government and Russia-backed rebel forces.

By the time we arrived in the area, ambulances had already taken away the dead and injured, but signs of the deadly attack were everywhere. Virtually all windows for several blocks were shattered. Firemen were still extinguishing fires in a couple of stores. A burned-out minivan was standing on the side of the street, and roads, backyards, and playgrounds were pock-marked by craters.

We counted more than 20 impact craters in the area, which spanned several blocks and included a market, apartment buildings, single-story houses, a church, and a school.

Human Rights Watch has previously documented how both sides have used Grad rockets in the war in eastern Ukraine. Evidence indicates that separatist forces were responsible for this attack: the Grad rockets struck government-controlled territory, the craters and rocket remnants stuck in the ground clearly indicate that the rockets came from due east, the direction of the frontline, and a rebel leader announced today that the rebel attack on Mariupol has begun in revenge for an attack Thursday that killed 13 people in a trolleybus in rebel-controlled Donetsk.

Grads are unguided rockets that cannot be targeted accurately, and are often fired in salvos from multi-barrel rocket launchers to saturate a wide area. Because of their indiscriminate nature, these rockets should never be used in populated areas, and their use in such areas is a violation of the laws of war.

Intensified fighting in east Ukraine during the last few weeks have involved increased violations of the laws-of-war and increased civilian casualties. With one of the attacks with the highest civilian casualties of the whole war taking place today, it is time for both sides to end the violations.