A suicide truck bombing in the restive Iraqi city of Ramadi has killed at least 35 people, police say.
Ramadi has been a centre of the insurgency in western Iraq
A police commander in Ramadi said a police checkpoint had been targeted.
Elsewhere, police said clashes had erupted between US and Iraqi forces and fighters from the Shia Mehdi Army militia in the city of Diwaniya.
A ban on vehicles has been imposed on the city following a raid by security forces at about 0500 (0100 GMT), police sources in the southern city said.
Police and Shia militia sources in Diwaniya said a large American force had launched an offensive in areas in the centre of the city where there is a strong presence of the Mehdi Army, the militia loyal to the radical cleric, Moqtada Sadr.
Heavy fighting was reported, with casualties among civilians as well as combatants.
A number of American vehicles have been destroyed in the fighting, police sources told the BBC.
There have been a series of sectarian killings in Diwaniya - which is 180km (110 miles) south of Baghdad - in recent days, a source in the governor's office said.
Meanwhile, a police commander in Ramadi said the lorry had been driven into a police checkpoint in the western part of the city and the bomb detonated.
Many of the dead were women and children, a police officer told the AFP news agency.
"The truck contained many tons of chlorine and TNT which were covered by sacks full of fertiliser," he said.
There are reports that several policemen were killed. Fifty people were reportedly injured in the blast.
The bombing was in a densely populated district and there are fears many of the casualties are civilians.
Some reports said the bomb had released chlorine gas and that 20 other people were treated for chlorine poisoning. The Associated Press (AP) news agency said most were discharged after 30 minutes.
There have been 10 bombings in Anbar province this year using chlorine gas as a weapon.
Ramadi has been a centre of the Sunni-led insurgency against coalition forces and the Iraqi government.
Two bombs were founds on a road frequently travelled by US and British diplomats near the southern city of Basra, AP reported.
It followed the deaths of four British soldiers in a roadside bomb blast in Basra on Thursday.