More than 3,000 US troops have arrived in Baghdad, the first deployment of extra forces promised for the Iraqi capital by US President George W Bush.
US and Iraqi forces are preparing for a security drive in Baghdad
As the deployment began, the US military said four soldiers and one marine had been killed in the restive western province of Anbar.
It took to 25 the number of US deaths in Iraq on Saturday - one of the worst days for US troops since the invasion.
In the latest violence in Baghdad seven people were killed in two blasts.
A bomb on a minibus killed six people in Karrada, a mostly Shia district. The second bomb hit central Baghdad.
In the south of the country, a British soldier was killed by a roadside bomb near the city of Basra.
The 3,200 extra troops sent to Baghdad are the advance guard of a 21,500-strong deployment ordered by President Bush earlier this month.
Saturday was the deadliest day for US troops in Iraq since January 2005, when 36 service members were killed. In addition to the deaths in Anbar:
- Twelve US troops were killed in a helicopter crash near Baghdad
- Five US soldiers were killed in a clash with militants in the Shia holy city of Karbala
- Three other soldiers were killed in separate attacks across the country
The extra US troops being sent to Baghdad are meant to bolster Iraqi efforts to combat the continuing bombings, killings and sectarian violence.
Mr Bush has admitted that the battle for Baghdad will prove crucial to the outcome of the conflict in Iraq, but his plans are opposed by political rivals in the US.
Meanwhile the political movement headed by radical Shia cleric Moqtada Sadr has said it is ending a two-month boycott of Iraq's parliament and government.
The boycott was called in protest at a meeting between Prime Minister Nouri Maliki and President Bush and to press for the withdrawal from Iraq of US troops.
Mr Maliki is politically dependent on support from Mr Sadr's bloc, but he has vowed to crack down on Mr Sadr's Mehdi Army as part of the planned security drive in Baghdad.