Militants have closed all gates of a Euphrates River dam they control in Iraq, blocking a major water source, a minister said on Monday, while violence killed 15 people.

The latest unrest comes amid a protracted surge in nationwide bloodshed that has claimed more than 2,550 lives so far this year and sparked fears of Iraq slipping back into the all-out sectarian killings of 2006 and 2007.

The unrest has been driven principally by widespread anger among the Sunni Arab minority over claims of mistreatment at the hands of the Shiite-led government and security forces, as well as by the civil war in neighbouring Syria.

Militants have "completely closed the gates of the Fallujah dam since yesterday (Sunday) morning," Water Resources Minister Muhanad al-Saadi said in a statement.

The move blocks a major source of water for central and southern Iraq.

The militants, who seized the dam several weeks ago, had previously cut the flow of water through the dam near the city of Fallujah, just a short drive west of Baghdad, but reopened it when water accumulated and caused the area to flood.

In a sign of both the reach of anti-government fighters and the weakness of security forces, all of Fallujah and shifting parts of Anbar provincial capital Ramadi, to its west, have been out of government control since early January.

The US embassy issued a statement Monday condemning "ongoing terrorist acts" by powerful jihadist group the Islamic State of Iraq and the Levant (ISIL), and the dam closure in particular.

"Targeting dams and other vital infrastructure victimises innocent Iraqi citizens. In the past week, hundreds of thousands of innocent Iraqis have suffered from water shortages as a result of ISIL's actions," the embassy said.

It added that the US has provided Iraq with "essential military equipment," and "will continue to accelerate such deliveries in order to ensure Iraqi security forces are equipped with modern and effective weaponry appropriate to the serious threat that ISIL poses."

Violence in various areas of Iraq left 15 people dead on Monday.

Shelling and shoulder-fired rockets killed two people and wounded seven in Fallujah, while clashes in Ramadi left five militants dead.

Bombings in three areas close to Baghdad killed five people, among them two Sahwa anti-Al-Qaeda militiamen, and wounded nine.

And north of the capital, a firebomb thrown at a checkpoint killed a policeman in the city of Tikrit, while gunmen killed a Kurdish security forces member and a civilian in Kirkuk.

Iraq's security forces will face a major test on April 30, when Iraqis go to the polls for the first parliamentary election since American forces quit the country at the end of 2011.

While they were able to keep violence to a minimum during provincial polls last year, the security forces have failed to halt a subsequent year-long surge in unrest.