US and British forces have thrust into southern Iraq, as Baghdad was rocked by the second fierce aerial bombardment since the start of the war to oust Saddam Hussein.
US planes have been in action overnight
As American units pushed northwards, British marines backed by tanks mounted an amphibious assault on the strategically important Al Fao peninsula in south-eastern Iraq.
But the First US Marine Expeditionary Force met quite stiff resistance from Iraqi forces, soon after it crossed the Kuwaiti border at daylight, reports the BBC's Adam Mynott, who had to take cover under fire.
Twelve British and four American personnel were killed when a CH-46 Sea Knight transport helicopter crashed in Kuwait.
The deaths are the first western fatalities since the conflict began, although the crash is believed to have been an accident.
Elsewhere in southern Iraq, the BBC's Clive Myrie saw scores and scores of Iraqi soldiers surrendering to 40 Commando, UK Royal Marines.
Despite two punishing air raids within 24 hours on Baghdad, US officials said the attacks were not the "shock and awe" bombardments which the Pentagon has said it will mount.
Shortly after the troops crossed into Iraq, the Kuwaiti news agency, Kuna, said US and British forces had captured the southern Iraqi city of Umm Qasr, a key oil shipping terminal, although Iraqi television denied this.
As the ground campaign unfolded, Australian defence officials said their special forces were operating deep inside Iraq, identifying troops movements and military targets.
Australian FA-18 Hornets - part of a 2,000-strong Australian force sent to the Gulf - have also been protecting US aircraft on missions over Iraq, the Australian defence minister said.
Hours before the second air attack on Baghdad, Iraq fired 10 missiles into Kuwait, without causing any casualties.
In his first public appearance since America launched its first strike, President George W Bush praised the US troops in action, saying: "There's no question we've sent the finest of our citizens into harm's way."
Iraqi television also broadcast what it said was a new statement from Saddam Hussein.
An announcer read the text, in which the president spoke of Iraq's steadfastness in the face of falsehood and criminals.
In the Iraqi capital, huge explosions were heard during the raid amid Iraqi anti-aircraft fire.
The BBC's Paul Wood, who is in Baghdad, says the explosions were much heavier and closer to the centre of the city than Thursday morning's attack.
Saddam Hussein's main presidential palace and the ministry of planning were struck.
Our correspondent says that the former office of Iraq's Deputy Prime Minister Tariq Aziz was among sites hit.
Iraqi television said US forces have fired 72 cruise missiles since the start of the attacks.
It said four Iraqi soldiers have died and five have been wounded.
The International Red Cross said one person was killed and 14 others were injured in the first attack.
There is no word of casualties from the second assault.
US officials said the operations did not yet represent the massive bombardment warned of by US Defence Secretary Donald Rumsfeld.
Mr Rumsfeld said: "What will follow will not be a repeat of any other conflict."
"It will be of a force and scope and scale that has been beyond what has been seen before."
He said damage assessment of targets struck in Baghdad in the first wave of attacks was still "pending", and repeated US advice to Iraqi troops to defect.
The BBC's David Willis, who is with the US marines in southern Iraq, said troops are snaking their way through the desert, heading for Iraq's southern oil fields to protect them against attack from Saddam Hussein's forces.
The 10,000 vehicle-strong American Third Infantry Division poured into Iraq after launching a sustained bombardment of Iraqi positions from the Kuwait-Iraq border.
More than 100 artillery shells were fired in the direction of southern Iraq, while Black Hawk helicopters swept over the desert ahead of the troops and tanks.
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British Royal Marines also joined the ground offensive, firing AS-90 guns some 15 kilometres (nine miles) into southern Iraq.
The US 1st Marine Division entered Iraq at about 2100 local time (1800 GMT), encountering some resistance from Iraqi troops.
They are reported to have destroyed an Iraqi T-55 tank.
Three Commando Brigade of the Royal Marines, meanwhile, landed at the Al Fao peninsula.
Correspondents say their destination is unclear, although it could be the southern city of Basra.
Large explosions were reported from the direction of Basra, 70km (44 miles) north of the Kuwaiti border, and unconfirmed reports say several oil wells in the area are on fire.
The Arabic television station al-Jazeera also reported explosions in the northern city of Mosul.
In other developments:
More than 1,000 people are arrested at an anti-war demonstration in San Francisco, while similar protests against the conflict are held throughout the world.
- UK Prime Minister Tony Blair addresses the nation; he says he is aware that many British people are against the war, but it is necessary and justified.
- Israel radio says US forces are operating in western Iraq to try to thwart the potential for missile attacks on the Jewish state.
- United States expels remaining three diplomats in country, announces it will freeze all non-diplomatic Iraqi Government assets and asks other countries to take similar action
- Parliament in Turkey votes to allow US planes to use Turkish airspace for the war on Iraq
The BBC's Tim Franks in northern Kuwait says the country has come under fire from 10 Scud-type missiles, some of which are reported to have been destroyed mid-flight by Patriot interceptor missiles.
Military sources in Kuwait say that two or three missiles were aimed at a Kuwaiti oil refinery.
Targets also apparently included Kuwait City and the British and American troops who had assembled near the Iraqi border.
One of the missiles landed just 300 metres from the US camp.
All were carrying conventional warheads, but US and British forces were taking no chances, donning gas masks and protective gear each time the air raid sirens sounded, our correspondent said.
Iraq has denied firing missiles into Kuwait.
BBC News Online's world affairs correspondent Paul Reynolds says that if it were confirmed that Iraq had fired Scuds, this would be proof that Iraq had broken the terms of the UN resolution which bans Iraq from having missiles with a range beyond 150km (93 miles).