Overall violence is well down on the levels of several years ago
US forces in Iraq suffered their worst casualties last month since September 2008, with 24 soldiers killed.
That brings the total number of US casualties since the 2003 invasion to just over 4,300.
US forces are due to be off the streets of cities and main towns by the end of June, while combat operations across Iraq are due to end by September 2010.
Civilian deaths from violence in Iraq fell sharply in May, according to Iraqi government figures.
The defence, interior and health ministries gave a civilian toll of 124, down from 355 in April, AFP reported.
US President Barack Obama has pledged to remove all US troops from Iraq, other than those involved in training, by the end of 2011.
Although overall violence is sharply down on the levels of several years ago, the surge in US military deaths in May has left many here feeling uneasy about what will happen as the Americans begin their pullout, the BBC's Nicholas Witchell in Baghdad says.
But Iraq's leaders are confident and say Iraqi forces are now quite capable of handling internal security without US support, he says.
In September 2008, 25 US service personnel were killed - the highest recent toll for US forces.
The figures take into account both combat and non-combat deaths.