Buildings and television stations in the centre of the Iraqi capital Baghdad have been hit in dawn raids by US-led coalition forces.
Thick smoke can be seen from a burning building in Baghdad
And at least 40 explosions were reported south of the city, where Iraqi leader Saddam Hussein's Republican Guard is reported to be dug in.
UK troops are engaging Iraqi paramilitaries fleeing the southern city of Basra - where the civilian population was reported to have staged an uprising against the government of Saddam Hussein.
However, latest reports from Basra appear to indicate that the streets are quiet.
Further north, US Marines have continued to meet guerrilla-style resistance and sniper attacks in and around the city of Nasiriya - a key crossing point on the Euphrates for the Marine advance on Baghdad.
Reuters news agency says coalition air strikes have been called in to support Marines facing determined Iraqi resistance near the town of Ash Shatrah, 40 kilometres (25 miles) north of Nasiriya.
Tuesday saw US troops in forward positions near the town of Najaf engaged in clashes with Iraqi forces, which Pentagon officials said left between 150 and 300 Iraqis dead.
The BBC's Nick Childs at the Pentagon says this appears to have been an intense engagement, but did not appear to be the start of the push on Baghdad.
A defence official said Iraqi ground forces had tried to hit US forces of the US 7th Cavalry with rocket-propelled grenades, but there are no reports of US casualties.
Wednesday's resumed raids on Baghdad came after sandstorms, which had hampered some coalition operations, eased.
US Central Command in Qatar has said that missiles struck Iraq's main TV station as well as a key communication point vault early on Wednesday, damaging Iraq's command and control capability.
The Pentagon said the purpose of the operation was to counter command and control abilities of the Iraqi regime, and also to deal with propaganda and the disinformation campaign of Baghdad.
But British Defence Secretary Geoff Hoon said no direct attempt had been made to take Iraqi television off the air.
The headquarters of state television, which was off air at the time of the raids, has been left a smouldering wreck but programmes resumed at about 0900 (0600GMT).
Iraq's international satellite channel, which stopped broadcasting at the time of the explosions in the city, still appears to be off the air.
A B-52 bomber took off from RAF Fairford in western England at 0337GMT on Wednesday, indicating that more bombing raids on the Iraqi capital could be imminent.
In fighting elsewhere:
- Two British soldiers were killed and two seriously injured when their Challenger tank was hit in a "friendly fire" incident during fighting early on Tuesday on the outskirts of Basra
- After securing the route through the town of Nasiriya - about 370 km (230 miles) south-east of Baghdad - US Marines seize a hospital that appears to have been used by Iraqi forces to store weapons
- British Royal Marines move into positions along the Iraqi border with Iran, amid worries that Iran might try to exploit the chaos caused by the war
The Al-Jazeera satellite channel has reported bombing raids on the northern city of Mosul early on Wednesday.
The BBC's Jim Muir in northern Iraq said coalition troops also appeared to be being flown into the region at night in preparation for what could be a the start of a northern front.
He said some bombing raids may also have been carried out on the Ansar al-Islam, the radical Islamic faction which the US says has links to the al-Qaeda network, in the hills north of Halabja.
The 800 or so Ansar al-Islam are expected to be the target of a ground offensive by Kurdish peshmerga - or volunteer - guerrillas working in co-ordination with the US special forces, he said.
Major General Peter Wall, British Chief of Staff at Allied Central Command in Qatar, said the Basra uprising was in its infancy and British troops were "keen to exploit its potential".
According to military intelligence officials, Iraqi troops in the city turned mortar fire on their own civilians in an attempt to crush the unrest, but Baghdad denies any revolt has occurred.
Coalition planes also dropped two very large bombs on the Baath Party headquarters in the city, which is reported to have been reduced to rubble.
The United Nations has warned of a potential humanitarian crisis in Basra, which is home to about 1.5 million people.
Some 100,000 children are at risk of disease as fighting there has continued for four days, disrupting supplies of drinking water, a UN spokesman said.
A UK naval ship loaded up in Kuwait with stocks of food, water and other supplies is still waiting to dock in the nearby Iraqi port of Umm Qasr as work continues to clear mines from the sea there.
US President George W Bush and UK Prime Minister Tony Blair are due to meet on Wednesday to discuss the immediate future of Iraq if Saddam Hussein is deposed.
The role of the UN in a post-war Iraq could be a sticking point at the talks.
The BBC's Washington correspondent Justin Webb says that unlike Mr Blair, the Bush administration has not been convinced of the need for the UN to be involved in setting up a post-war Iraqi government.