Eight British and four American troops died in a helicopter crash as the ground invasion of Iraq began.
The British troops from 3 Commando Brigade and US troops were killed when a US CH-46 Sea Knight helicopter crashed in Kuwait several miles south of the Iraqi border said US defence officials.
Defence Secretary Geoff Hoon told the Commons an investigation had begun into the crash but it was not the result of "enemy action."
US and British forces have advanced deep into Iraqi territory.
Prime Minister Tony Blair said Britain owed an "immense debt of gratitude" to the troops who had died.
"These were brave men who in order to make us safer and more secure faced the risks and had the courage to serve the country and the wider world," he said.
Eight B-52 bombers took off from RAF Fairford in Gloucestershire shortly after 1000 GMT.
A recovery team is at the site of the crash which happened at about 0300 local time - 0000 GMT - close south of the southern Iraqi city of Umm Qasr.
Mr Hoon confirmed the helicopter had been engaged in an operation led by 3 Commando Brigade on the al-Faw peninsula in south-eastern Iraq.
The southern tip of the peninsula was secured in an air and sea assault by 40 Commando Royal Marines without damage to oil installations, he said.
MoD Helpline Numbers
Royal Navy and Royal Marines - 08457 414544
Army personnel - 01980 615500
RAF personnel - 01452
712612 ext 7080 or 7045
A former US Airforce pilot told the BBC he thought the crash was probably caused by the age of the aircraft, which had been service for almost 40 years.
The Royal Navy confirmed some of the troops killed were from their Plymouth base but details will not be released until next of kin have been informed.
An elite fighting force, the 3 Commando is made up of 40, 42 and 45 Commandos.
As UK troops fought their way north possibly towards the port of Basra they faced opposition at the key oil shipping terminal Umm Qasr, according to Mr Hoon.
He told MPs troops had met some resistance with a small scale engagement resulting in four known Iraqi casualties.
He said the Iraqi regime had set light to up to 30 oil wells but the operation aimed to stop further similar destruction.
Scores of Iraqi soldiers surrendered to British forces at al-Faw, according to BBC correspondent Clive Myrie.
US Marines have reached Iraq's only deep-water port at Umm Qasr in the south-east but BBC correspondents say they have not yet gained control of the whole complex.
By lunchtime BBC correspondent Hilary Andersson said about 4,000 of the UK's Desert Rats were crossing into Iraq.
Overnight RAF Tornado GR4 and Harrier jets flew sorties from Kuwait to support US ground troops in the Basra operation, as well as other operations within Iraq.
The US Marines use the Sea Knight, a helicopter with two large rotors like a Chinook, to fly troops from ships at sea or base camps to forward positions.
Colonel Walter Boyne, a former US Airforce pilot, told the BBC he thought the crash was probably caused by the age of the Vietnam-era aircraft.
The US Navy and Marines grounded all 291 Sea Knight helicopters in August after an inspection of a helicopter in North Carolina found a crack in a rotor assembly.
"I just wish that we had been vigilant enough and foreseeing enough to go ahead and re-equip with helicopters," Mr Boyne said.