US troops in Iraq lost 77 men in May – the highest toll since January - with the US military admitting that daily attacks have doubled in recent weeks.
The latest casualty was in Ramadi, where the US Central Command said a marine assigned to the 5th Marine Regiment was killed in action on Monday by enemy small-arms fire while conducting combat operations.
The latest spree of violence put an end to a brief lull when attacks on US forces had waned after the 30 January elections in Iraq.
The toll in May was the highest since 107 Americans were killed in January.
Lieutenant Colonel Steve Boylan, a US military spokesman in Baghdad, said anti-US fighters were staging about 70 attacks across Iraq per day.
US soldiers have failed to quell
violence in Iraq
"There was a lull in attacks after the elections," Boylan said.
"There was a period of time right after the election until the beginning of April or middle of April that we actually saw them dip into the low 30s."
In the recent spike in violence, fighters also have aggressively targeted Iraqi security forces. Boylan said more than 600 Iraqis were killed or wounded in May.
May was a record month for car bombs used by fighters. US forces also suffered losses in offensives against the fighters such as Operation New Market in the western town of Haditha and Operation Matador around the western town of al-Qaim.
Boylan could not say whether the fighters could sustain the current level of violence.
Defence analysts said the recent violence was the latest evidence that Iraq remained an uncertain project for the United States.
"Those who believed that the elections would be a decisive turning point undermining the insurgency are disappointed yet again," Cato Institute defence analyst Ted Carpenter said.
"The insurgency seems as capable as ever."
US generals in the weeks after the election had talked about a possible serious reduction in US troop levels next year.
"The reality is we have discovered, despite all our propaganda, that we are facing a very tough, resilient and smart adversary"
Defence analyst of the Lexington Institute
"The reality is we have discovered, despite all our propaganda, that we are facing a very tough, resilient and smart adversary," defence analyst Daniel Goure of the Lexington Institute said.
But Boylan preached patience.
"This is the hardest type of fight to be in," Boylan said. "If we get too impatient and decide to throw in the towel too soon, then we give up everything we have gained up to this point."