Small, highly mobile units picked from the US Army’s most revered and secretive fighting force have been assigned a key mission of the war: to hunt down Saddam, his two sons and at least a “dirty dozen” of Iraq’s top military and civilian leaders.

The Delta Force, the US equivalent of the British SAS, has 306 men. It has been training for several years with the CIA for the specific mission of hunting down the Iraqi leader, officials said.

Last night they were being mobilised to infiltrate Baghdad and Saddam’s home city of Tikrit to begin the hunt.

As plans were revealed to drop the commandos from Black Hawk helicopters to sites outside Baghdad, it became clear that, if US forces locate Saddam, the likelihood is that they will kill him and his closest henchmen rather than capture them.

“The expectation is to kill him within days (of the start of the war),” a Pentagon official said.“It’s what Delta has been training 24/7 to do.”

Assassinating a foreign leader runs counter to a 1976 order signed by President Ford. But White House officials cite international law, which states that, once a war begins, there are no limits on military actions against enemy leaders. Saddam, as Commander-in-Chief of Iraq’s Armed Forces, is a legitimate target, they say.

CIA operatives have been photographing and spying on Saddam’s numerous presidential compounds, while US spy satellites take daily pictures of the Iraqi leader’s suspected hideouts. Some of the most detailed information on his possible whereabouts, Pentagon officials said, has come from Jordanian intelligence.

Saddam will prove a highly elusive prey, however. During the 1991 Gulf War, allied aircraft bombed 260 “leadership targets”, including underground bunkers, command centres and offices, but failed to touch him. The Iraqi leader, who has at least three surgically enhanced body doubles, spent 38 nights of Operation Desert Storm hiding in the homes of ordinary families, never staying in the same place twice, a tactic that he is likely to repeat.

He also claims to have more than 400 hideouts in Baghdad, homes and apartment buildings indistinguishable from ordinary Iraqi residences. The US commandos are preparing to conduct house-to-house searches for him, officials said.

In Baghdad, a city of more than five million people, Saddam has a dozen presidential compounds, connected with tunnels built by Yugoslav experts who built a network of underground hideouts for Marshal Tito during the Cold War. Saddam also has several unmarked lorries in which he can live for days at a time.

The Iraqi leader views himself as an indefatigable survivor, having emerged unscathed from at least seven domestic assassination attempts, a CIA-sponsored coup attempt and the 1991 Gulf War.

The first job of Delta Force commandos will be to isolate Saddam from his military commanders. They plan to hack into and shut down Iraq’s communications and power facilities using laptop computers. They hope to prevent Saddam from communicating with senior officers who might help him to escape or might be awaiting orders to use chemical or biological weapons.

Pentagon officials concede that the best chances of finding him lie with informants. “We’ve been trying to track Saddam down since the beginning of the Gulf War, without success,” retired Marine Lieutenant Gregory Newbold said.

Peter Singer, a Washington-based Iraqi expert said: “He carries a gun, is always surrounded by armed bodyguards and will not want to be captured. The order is not to risk US lives to capture him alive. But it could take a very, very long time to find him.”

If Saddam is captured, he will be tried for war crimes, although the forum has not yet been decided on. The likely choice is between the International Criminal Court in the Hague, a body the United States has refused to recognise, or a military tribunal set up by America.

One advantage of a successful prosecution is the damning evidence that would emerge and would justify the invasion.

Special operations forces will play a support role to conventional ground forces in Iraq. But it will still be a much bigger part than they played in the first Gulf War, when they were introduced, almost as an afterthought, after Iraq began firing Scuds into Israel and Saudi Arabia.