A roadside bomb exploded near an Iraqi National Guard patrol south of Baghdad on Tuesday, killing three guardsmen and wounding 11, an official said.
The guardsmen were patrolling an area near Jibala, 60km south of Baghdad, when the attack occurred.
Also on Tuesday, a US marine assigned to the 1st Marine Expeditionary Force died in what the military described as "a non-hostile motor vehicle accident" in al-Anbar province.
Meanwhile, the US military headquarters in al-Kadhimiya district north of Baghdad came under mortar attack, Aljazeera said.
Columns of smoke were seen rising from the site. There were no immediate reports on casualties resulting from the attack.
The CIA has cautioned that
security might not improve soon
The violence, including a roadside bomb attack on a US patrol in Baghdad on Monday that wounded one soldier, persisted despite offensives intended to suppress anti-US opposition before January's elections.
The US military said its troops captured 14 Iraqis, including 10 wanted for making explosive devices.
Seven, captured in al-Siniya, about 250km north of Baghdad, on Monday, were alleged members of a car bomb-making cell.
Another seven, including three accused of bomb-making, were captured in Tikrit, Saddam Hussein's hometown.
In the western city of Ramadi, fighters on Tuesday ambushed an Iraqi police patrol, robbing them of their weapons and burning their vehicles but letting the officers go, Ramadi's police chief said.
US troops are out in force in
the northern parts of Ramadi
The attack occurred in the centre of the city, 110km west of Baghdad shortly after midday, police chief Brigadier Jasim al-Dulaimi said.
Fighters have repeatedly attacked Iraqi police and National Guards in recent months, part of an apparently coordinated strategy to undermine the country's inexperienced security forces before elections due at the end of January.
A reporter working for Reuters in the city said there was a heavy presence of US troops on Tuesday in northern sections of Ramadi, where palm groves and extensive farmlands have in the past been used to launch attacks on US forces.
The situation in Iraq is unlikely to improve anytime soon, according to a classified cable and briefings from the CIA, the New York Times reported on Tuesday.
The assessments are more pessimistic than the Bush administration's portrayal of the situation to the public,
government officials told the newspaper.
The classified cable, sent last month by the CIA's station chief in Baghdad after the completion of a one-year tour of duty there, painted a bleak picture of Iraq's politics, economics and security, and reiterated briefings by senior CIA official Michael Kostiw.
The cable, described as "unusually candid", cautioned that security in the country was likely to deteriorate unless the Iraqi government made significant progress in asserting its authority and building up the economy.
New role for troops
The top US commander in the Gulf region sees the role of US troops in Iraq possibly shifting as early as next year to focus less on combat and more on training Iraqi security forces, the Washington Post reported.
John Abizaid is especially worried
Army General John Abizaid did not set a timetable for the shift, saying it would depend on the outcome of the January elections and evidence Iraqi forces were ready to handle the fighters, the newspaper said.
The US Central Command chief was quoted as saying the reshaping would include such changes as "more embedded trainers" and possibly more special operations forces replacing conventional ones.
According to the Post, other senior US officers said Abizaid's plan would not necessarily lead to an immediate reduction in US troop levels, but could pave the way for an eventual reduction.
The Pentagon announced last week that it would boost its troop level in Iraq to 150,000 this month, the highest level since the Iraq war began in March 2003, in order to improve security for scheduled 30 January elections.
The Post said Abizaid admitted being especially worried about the situation in Mosul, where most of the city's police fled after an onslaught by anti-US fighters last month.