Casualties from cluster munitions, internationally outlawed weapons that kill indiscriminately, more than doubled in 2016 compared with the previous year, a disarmament group that monitors their use reported on Thursday.

The group, the Cluster Munition Coalition, said nearly 1,000 people had been killed or injured by the bombs, which are banned by international treaty. Most of the increase was attributed to the six-and-a-half-year-old war in Syria, where use of cluster munitions by the military was documented early in the conflict and appeared to have escalated last year with Russia’s support.

Dozens of other deaths and injuries last year were attributed to the Saudi-led bombing of Yemen and lethal remnants buried in Laos from the intensive American bombing of that country during the Vietnam War era, the group said.

The findings were contained in the Cluster Munition Monitor, the group’s annual report on adherence to the Convention on Cluster Munitions, the treaty that prohibits them. The treaty entered into force on Aug. 1, 2010. To date, 119 countries have signed the treaty or agreed to its provisions.