The US on Monday urged Iraq to investigate the "abhorrent" killing of protesters, condemning excessive force in the flashpoint southern city of Nasiriyah.
"The use of excessive force over the weekend in Nasiriyah was shocking and abhorrent," said David Schenker, the US diplomat in charge of the Middle East.
"We call on the government of Iraq to investigate and hold accountable those who attempt to brutally silence peaceful protesters."
Security forces killed at least 45 civilians who were protesting in the city on Thursday, in one of the bloodiest incidents since the outbreak of the anti-government protests.
The government intended to show brute force after the Iranian consulate was set alight on Wednesday.
Southern Iraqi tribal fighters have intervened to calm and secure the city.
Iraq's judiciary on Sunday issued an arrest warrant against an army general over the killing of protesters in the southern Iraqi city.
Iraq has been rocked by weeks of protests against corruption and poor public services that led on Sunday to Parliament accepting prime minister Adel Abdul Mahdi’s resignation two days after he announced his intention to stand down.