Ten US troops have died in clashes with insurgents or other combat in Iraq this weekend, the US military says.
US impatience with rebel Shia cleric Moqtada Sadr is growing
The dead include five marines killed in pitched battles near the Syrian border and three soldiers ambushed near a city south of Baghdad.
Marines killed 25 to 30 insurgents in the battle, the US statement claimed.
Nearly 100 American troops have now been killed in April, the bloodiest month for the US since the invasion of Iraq more than one year ago.
Meanwhile, a tense stand-off is continuing in the holy city of Najaf between US forces and supporters of radical Shia cleric Moqtada Sadr. A spokesman for Mr Sadr said all mediation efforts have so far failed.
The Americans insist Mr Sadr must be captured or killed and his militia disbanded, but so far they have respected Shia calls not to attack the city.
The BBC's regional analyst Sadeq Saba, says it is hard to believe US troops can afford to wait at the gates of Najaf for long.
Najaf is one of the most sacred cities for the world's Shia Muslims
He says sovereignty is due to be transferred to Iraqis by the end of June and the Americans are keen to end rebellions by militant groups.
Najaf is one of the most sacred cities for the world's Shia Muslims, where one of the major figures of the faith, Imam Ali, is buried.
If US military planners make a deliberate decision not to attack the city, armed Sadr supporters could turn it into their stronghold for resistance.
But should the US decide to enter the city it could run the risk of inflaming the Shia world, especially if the Imam Ali shrine is damaged.
The US army said on Sunday 10 US troops had been killed in combat across Iraq - and an 11th had died when a tank rolled over.
The US confirmed five marines were killed in clashes with insurgents near the Syrian border, after their patrol was ambushed on Saturday morning.
The marines fought "a day-long series of firefights" with 120 -150 insurgents outside Qusayba, near Qaim, the release said.
"Additional marines, backed by helicopter close-air support,
were dispatched to the city and soon came under fire by enemy
equipped with rifles and RPGs," it added.
The US army said marines killed 25 to 30 insurgents in the battle, but a local hospital
official reported 10 Iraqis killed.
In a separate statement, the US military said: "Three soldiers travelling in a 1st Armoured Division convoy were killed during a small arms ambush" near the southern city of Diwaniya around 1700 [1500 GMT] on Saturday.
Another US soldier was killed on Saturday morning when a
roadside bomb exploded near a military convoy in Baghdad, the military said.
Separately, a US marine from the 1st Marine
Expeditionary Force was also killed in action in western Iraq, the US army said.