President Bush braced Americans for more torrid weeks of killings, kidnappings and ambushes in Iraq, but insisted that failure was "unthinkable".

"We must not waver," Mr Bush told a television audience of tens of millions of Americans in a prime-time news conference.

He conceded that coalition forces had been through "tough weeks". But he vowed that there would be no slipping from the June 30 date for the handover of sovereignty to fledgling Iraqi authorities. Any retreat on the battlefield or from the handover deadline would embolden America’s enemies, he said.

"The work may become more difficult before it is finished. No one can prevent the hazards that lie ahead," he said from the East Room of the White House.

"In this conflict there is no safe alternative to resolute action. The consequences of failure in Iraq would be unthinkable."

He added: "I have directed our military commanders to make every preparation to use decisive force, if necessary, to maintain order and to protect our troops."

The event, which Mr Bush opened with a statement running to nearly 20 minutes, came as White House strategists worked feverishly to prevent the mounting US casualties from casting doubt on Mr Bush’s re-election hopes.

It came as US officials in Baghdad provided a graphic illustration of the reversals on the ground in the past two weeks, saying that 40 individuals from 12 countries were being held hostage by various groups.

US officials were examining four mutilated bodies recovered from shallow graves which were believed to be some of the seven Americans missing in Iraq.

It is thought they may be those of private contractors missing since an assault on their convoy outside Baghdad amid the spiralling of kidnappings in Iraq.

One of the seven missing employees - Thomas Hamill, a 43-year-old truck driver - is known to have been abducted. His captors have threatened to kill and mutilate him unless US troops ended their assault on Fallujah.

A French journalist and four Italians working as private guards were also reported abducted yesterday. The kidnapping of three Russians and five Ukrainians this week has prompted Russia to send planes to evacuate more than 800 of its nationals and citizens of the former Soviet states.

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