RAF Tornados have taken part in a series of massive air strikes on Baghdad using the RAF's new cruise missiles for the first time.
The Tornados were attacking radar defences
The Tornado GR4s were said to have targeted the radar defence systems that protect Saddam Hussein and his high command.
Once radar defences were damaged, other aircraft were following to deliver their payloads.
The huge increase in bombing was described by a military source as "one of the biggest nights of the war".
The Tornados - launched from IX(b)Squadron at 1700 GMT (2000 local time) - were using Air Launched Anti-Radar Missiles (ALARM), which can lock onto radar systems which would otherwise help down coalition aircraft.
They were followed by a second wave of Tornados from 617 Squadron (The Dam Busters).
The squadron, normally based at RAF Lossiemouth, used the new Storm Shadow missiles to attack command centres near Baghdad.
The bunker-busting missiles are not yet officially in service and have only been tested twice before.
Group Captain Simon Dobb, detachment
commander at the Ali Al Salem base in Kuwait, said: "Our task is to take out the integrated air defence systems around Baghdad and thereby cause maximum isolation of Saddam Hussein's leadership and his infrastructures."
The coalition air offensive is expected to last for several days, with intense bursts of explosions targeted on the middle of Baghdad.
"We are expecting this to be on an unprecedented scale, far larger than Operation Desert Storm in 1991. It is the greatest night in the history of the
Tornado squadrons," added Group Captain Dobb.
Hours after the fresh bombing, the Ministry of Defence issued a statement which said: "Tonight has
seen the deliberate escalation of the campaign in Iraq through the aerial
bombardment of Baghdad."
Although it was a "significant" attack, it would be "carefully targeted against the regime" and minimise civilian casualties and infrastructure, the statement added.
Further details about the operation will emerge at a press conference on Saturday, by Defence Secretary Geoff Hoon and the Chief of Defence Staff, Admiral Sir Michael Boyce.
Also included in Friday evening's air assault were Tornado F3s and Harrier GR7s as well as Tomahawk cruise missiles launched from submarines HMS Splendid and HMS Turbulent.
And among the forces at the disposal of the US were eight enormous B-52 bombers, which left RAF Fairford in Gloucestershire at 1000 GMT on Friday.
'Debt of gratitude'
Sir Michael said earlier that American troops had taken complete control of the strategically important port town of Umm Qasr in southern Iraq.
He was speaking at a news conference in Whitehall minutes before air raid sirens sounded again in Baghdad ahead of the new wave of air strikes.
He said US troops flanked by two British battle groups had reached the outskirts of Basra, also in southern Iraq, on Friday.
He told the BBC: "With luck Basra might decide that it is not worth fighting ... and that coming forward peacefully will save a lot of people a lot of grief."
Sir Michael was speaking after Prime Minister Tony Blair paid tribute to eight UK commandos who died in a helicopter crash in northern Kuwait.
The British troops from 3 Commando Brigade were killed along with four US soldiers when an American CH-46 Sea Knight helicopter crashed in Kuwait several miles south of the Iraqi border.
Mr Blair said the accident underlined the dangers facing UK troops and the country owed the dead men "an immense debt of gratitude".
The UK says the crash was not caused by "enemy action".
Sir Michael said coalition troops would be able to use the deep-water port at Umm Qasr once it had been cleared of mines.
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He said the port had come into coalition hands after being "overwhelmed" by US Marines.
And Sir Michael said that the men of the UK's 40 Commando had taken the surrender of "very significant numbers" of Iraqi troops in the al-Faw
He said troops advancing on Basra had seen evidence that Iraqi forces had dumped equipment and fled.
The admiral said only seven southern oil wells had been set alight by the Iraqi regime - it had earlier been thought the figure could be up to 30.
Mr Hoon said fears that oil could be poured into the Gulf by the Iraqis had been quelled by the operation in al-Faw.