Shia militias battled Iraqi police for a second day running and bombs killed more than a dozen people as US President George W Bush talked of changing tactics.
The deaths of three US Marines on Saturday brings to nearly 80 the number of American troops killed in October alone.
If casualties continue to mount, this month will be one of the deadliest for US forces in Iraq since the 2003 invasion.
More than 2,700 US soldiers have been killed in Iraq since 2003.
'Our goal in Iraq is clear and unchanging: Our goal is victory. What is changing are the tactics we use to achieve that goal,' Bush said in his weekly radio address.
Worsening violence in the Shia heartland is testing the Shia-led government's ability to rein in militias and exposing a power struggle in the ruling Shia coalition that threatens to further complicate the US task in Iraq.
Bombs rigged to bicycles followed by a barrage of mortars killed 16 people and wounded 60 in a market in Mahmudiya, a town in the Sunni insurgent 'Triangle of Death' south of Baghdad, Interior Ministry sources said.
State television earlier quoted a local official as saying 30 died, but the ministry sources said their toll was 'final'.
