Sunni militants in Iraq have reportedly executed up to 190 captives in the northern city of Tikrit. Human Rights Watch reports the massacre took place earlier this month. The news comes as Iraqi Prime Minister Nouri al-Maliki faces increasing pressure to resign for failing to unite the country or beat back gains by the Islamic State of Iraq and Syria, or ISIS. Another round of 50 U.S. special operations troops have arrived in Iraq to act as “military advisers” on the crisis.