Cluster bombs, which have been used since World War II to kill and maim indiscriminately, were outlawed under an international treaty that was adopted in 2008.

Most of the world’s countries have signed the treaty, but not the United States and Russia, where many of the bombs were made. A report released on Thursday in Geneva found that the weapons continued to be used with near impunity in the conflicts in Syria and Yemen.

In Syria, at least 13 kinds of cluster munitions — dropped from the air or launched from the ground — have been used since 2012, said the report, the seventh annual Cluster Munition Monitor, which found “compelling evidence” that Russia had used the weapons since the start of its military intervention last September in support of Syria’s embattled government.

In Yemen, seven kinds of cluster munitions were used in 19 attacks from April 2015 to February of this year as part of the Saudi-led coalition’s fight against Shiite Houthi rebels, the report found. Saudi Arabia, which has not signed the treaty, has acknowledged using the munitions once, in April 2015. Most of the cluster bombs used in Yemen originated in three countries: the United States, Britain and Brazil.