GENEVA — Islamic State forces have committed genocide and other war crimes in a continuing effort to exterminate the Yazidi religious minority in Syria and Iraq, United Nations investigators said on Thursday, urging stronger international action to halt the killing and to prosecute the terrorist group.

The investigators detailed mass killings of Yazidi men and boys who refused to convert to Islam, saying they were shot in the head or their throats were slit, often in front of their families, littering roadsides with corpses. Dozens of mass graves have been uncovered in areas recaptured from Islamic State and are being investigated.

The investigators have produced 11 reports documenting wide-ranging crimes against humanity and war crimes committed by many parties to the five-year-old civil war in Syria, but in a report released on Thursday, they invoked the crime of genocide. They based their findings on actions taken by the Islamic State since August 2014 against 400,000 members of the Yazidi community, followers of a centuries-old religion drawing on many faiths.

“Genocide has occurred and is ongoing,” Paulo Sérgio Pinheiro, chairman of the panel, the Independent International Commission of Inquiry on the Syrian Arab Republic, said in a statement.