Iraqi government forces led by a U.S.-trained commander lost control of another key northern Iraqi city to Sunni extremists on Monday, while the United Nations' human-rights envoy said alleged massacres in the region over the weekend "almost certainly amount to war crimes."

The loss of Tal Afar, a city of around 200,000 people 270 miles northwest of the capital Baghdad near the Syrian border, underlined the fragility of Prime Minister Nouri al-Maliki's government and raised fresh questions about its ability to counter...