The United States is sending thousands more troops to secure the town of Nasiriya on the vital supply route to Baghdad.
US marines are attempting to overcome opposition in Nasiriya
Special forces soldiers as well as marines are being deployed to the area where coalition forces have come up against stiff resistance.
Invading US and UK forces say they are making advances elsewhere in Iraq, encircling the key central town of Najaf and seizing an opposition stronghold near Basra.
Coalition bombing of Baghdad is now almost constant, with correspondents saying the intensity of the raids has also increased.
Other key military developments:
- A convoy of more than 6,000 US marines resumes its push towards Baghdad and is preparing to confront a key unit of the Iraqi Republican Guard
- The Americans say their forces are fighting Republican Guards south of Karbala, about 80 kilometres (50 miles) south of Baghdad. US troops are reportedly battling for control of a key bridge over the Euphrates River
- US aircraft launch air strikes on Iraqi forces east of the Iraqi Government-held northern city of Mosul
Battle for Nasiriya
About 5,000 reinforcements are being sent to Nasiriya to support the 7,000 marines already fighting there, reports the BBC's Andrew North who is with a US unit near the city.
The US military has also been launching new air strikes on Nasiriya, as well as the nearby town of Suq al Shuyukh.
Our correspondent says fighting in the area has slowed supplies and the priority is to get things moving again.
He says the marines are also increasing their humanitarian operations in the city, to try to win over the local population.
The BBC's Adam Mynott, who is with another unit near Nasiriya, says the intention of the US commanders now seems to be to take control of the town to neutralise any opposition rather than trying to bypass it.
US marines have found stashes of weapons, ammunition and chemical protection suits at camps in the Nasiriya area, apparently abandoned suddenly by Iraqi forces, our correspondents say.
Further north on the invasion route to Baghdad, assault troops from the US 101 Airborne Division are reported to have surrounded Najaf, about 160 km (100 miles) south of the capital.
There was carpet-bombing of Republican Guard positions overnight by B-52s, the BBC's Gavin Hewitt says.
HUMAN COST OF WAR
Iraq: 589 civilian deaths, military deaths unknown
US: 38 dead (including 11 in accidents, 2 under investigation), 19 missing
UK: 25 dead (including 15 in accidents, 5 to 'friendly fire')
*Figures from each government
He says Iraqi forces may have attempted to break out during the night, leading to intense fighting.
It is not known whether the coalition forces plan to simply cordon off the city, or overrun it to ensure Iraqi fighters are not a danger to supply lines.
In Baghdad, the information ministry was set ablaze during overnight bombing - the second attack on the ministry in three days.
Other blasts hit a telephone exchange and a presidential palace.
The attacks were carried out simultaneously by multiple B-1, B-2 and B-52 bombers, US Central Command said.
According to the Americans, three quarters of the bombing efforts are now centred on the Republican Guard divisions to the south of the city.
BBC correspondents in Baghdad say people in the centre of the city appear to be coming to terms with the bombardment. There is more traffic and shops and restaurants are starting to reopen.
Large television aerials are reported to be selling well at the local markets.
State-run television was off air for a few hours on Monday following overnight missile strikes.
Iraqi Foreign Minister Naji Sabri has told reporters in Baghdad that the US-led forces are colonialists who aim to "partition" the country "not to create a unified Iraq".