At least 15 people have been killed in a suicide car bomb attack near the Iraqi city of Falluja, police and the US military said.
The US says there will be a "tough fight" this summer
At least 13 people were also wounded when the attacker detonated the car bomb in a market place, they said.
The attack came as the US admitted its security "surge" in and around Baghdad faced serious challenges.
A US security review said coalition forces controlled fewer than one-third of Baghdad's neighbourhoods.
One report said the market bomb in Amiriyah, in Anbar province, targeted elders of the al-Buissa tribe, most of whom are anti-al-Qaeda.
Police told the Associated Press news agency the bomb exploded as the elders met to try to resolve a tribal dispute in the town, 65km (40 miles) west of Baghdad.
In another incident, Iraqi police said they shot dead a suspected suicide bomber as she tried to detonate her bomb at a Baghdad recruitment centre. The shots triggered her explosive belt, police said.
Three police recruits received minor wounds from the explosion. The woman died.
The attacks came as the US military review of the four-month security surge said there were still serious threats in Sunni-Shia areas of west Baghdad.
More than 20,000 US reinforcements are being deployed as part of the surge.
A US military spokesman said it would not be possible to judge the success of the Baghdad security plan until all the extra units had been put in place.
"It's going to get harder before it gets easier," Lt-Col Christopher Garver said. "We know it's going to be a tough fight over the summer."
Details of the interim assessment included information that US and Iraq forces were in control of just 146 of Baghdad's 457 districts.
The report highlighted a failure of Iraqi police and army units to provide all the forces promised to carry out basic security tasks including manning checkpoints and conducting patrols.
May had the third-highest death toll of US soldiers, 127, since the US-led invasion to overthrow the government of Saddam Hussein in March 2003.
Thousands of Iraqis have been killed since February.
Correspondents say there was a significant reduction in the number of sectarian killings early on in the crackdown, but numbers have now risen again, with dozens of bodies being found in Baghdad almost every day.
US President George W Bush won a tough battle with opposition-controlled Congress to fund the crackdown and is under pressure to show progress or start bringing troops home.