Sixteen US and British soldiers were killed in a helicopter crash late on Thursday as troops swept into southern Iraq, putting pressure on the southern city of Basra, Pentagon officials said.
The CH46E Sea Knight transport helicopter crashed in Kuwait at around 0040 GMT on Friday. "There were 16 people on board, a mix of American and British troops. Initial reports indicate there were no survivors," the Pentagon said in a statement.
Iraqi leaders were also kept off balance with a second set of cruise missile strikes in Baghdad, while Pentagon officials kept a virtual blackout on the unfolding ground offensive in southern Iraq.
Reports said that US and British forces had taken the Gulf port of Umm Qasr and British Royal Marines commandos had landed on the Fao peninsula south of Baghdad.
Iraq denied the official Kuwait news agency's report that Umm Qasr had fallen.
"All I can say is that the pressure is continuing on the Iraqi regime and it will not be there in the period ahead. And we still hope that it is possible ... without the full force and the fury of a war," US Defence Secretary Donald Rumsfeld said.
However, he also warned that a much anticipated "shock and awe" air campaign would go ahead if necessary, saying that the Pentagon wanted to maintain "as much ambiguity as possible until we begin the big push."
"We clearly haven't made the big air push yet," he added.
The US 3rd Infantry Division, meanwhile, fired a barrage of rocket-propelled artillery into southern Iraq and then moved out across the border, a Marine officer said.
Across the border from Kuwait lies a major Iraqi oil field that the US military has promised to protect.
Defended by a Republican Guard division, the Basra area also is vital to Iraq's economy because it controls the country's oil terminals in the Gulf and only access to the sea.
Meanwhile, Tomahawk cruise missiles launched from British submarines struck at least three buildings in Baghdad, setting one ablaze, some 16 hours after Iraqi President Saddam Hussein and other senior leaders were targeted with F-117 stealth fighters and cruise missiles.
"That's to keep peoples nerves on edge," a US defence official said of the latest air strikes in Baghdad.
Meanwhile, missile attacks into northern Kuwait and reports that Iraqi forces set fire to as many as four oil wells indicated that Saddam has started his own campaign hours after coming under attack in Baghdad.
Rumsfeld confirmed at a Pentagon news conference that senior Iraqi leaders were the targets of the first raid of an all-out war to topple the Iraqi regime.
A US official said it was a residential compound where Saddam and his sons Uday and Qusay sometimes slept.
Iraqi television broadcast a tape of Saddam reading a speech vowing no surrender. US intelligence agencies were analysing it to try to determine whether it was made before the strike.
US officials discounted speculation that the man was a double.
Rumsfeld and General Richard Myers, chairman of the Joint Chiefs of Staff, acknowledged that the attack meant a last-minute change in US plans to take advantage of fragile intelligence.
Not to have seized the opportunity would have been a "terrible mistake," Rumsfeld said.
"What will follow will not be a repeat of any other conflict. It will be of a force and scope and scale ... beyond what has been seen before," Rumsfeld told a Pentagon news conference.
But Rumsfeld emphasized that the war's scope hinges on whether the Iraqi military and others in the regime abandon Saddam.
"The more of them there are, the greater the chance that the war will be limited and less broad," he said.
The United States was communicating with Iraqi military officials from units of the regular army, the elite Republican Guard and the Special Republican Guard, which is responsible for the defence of Baghdad and is reputed to be the most loyal to Saddam, he said.
Claiming Iraqis feared for their lives under Saddam, Rumsfeld predicted that people would switch sides once convinced the Iraqi leader is on the way out.
"We have broad and deep evidence that suggests that there are people going through that decision-making process throughout that country today, and that is a good thing," Rumsfeld said.
Rumsfeld warned Iraqi leaders and generals not to order the use of chemical or biological weapons, not to destroy dams and flood villages, or set fire to oil fields, adding that war crimes would be prosecuted.