A suicide bomb outside a mosque in northern Iraq and a string of attacks in Baghdad killed at least 37 people on Friday, a grim reminder of the insurgency’s continued ability to strike in spite of recent security gains.

The attacks, targeting Shia Muslims as they celebrated one of their biggest religious holidays, marked one of the deadliest days in Iraq since the nominal withdrawal of US troops from cities on June 30. Violence in the country increased slightly this spring, but July was among the least violent months since the 2003 US-led invasion.