The US Secretary of State, Condoleezza Rice, has said that America must not abandon its mission in Iraq.
Ms Rice acknowledged the US death toll in Iraq
She said the American public had to realise the consequences of ceding the country to "barbaric killers".
She warned that such a move would embolden every enemy of liberty and democracy across the Middle East.
BBC state department correspondent Jonathan Beale says the speech is part of a concerted effort to halt the US public's sliding support for the war.
Recent polls have suggested waning support for the way President George W Bush has handled the situation in Iraq.
In a speech to students at Princeton University, Ms Rice acknowledged the sacrifice being made by US forces in Iraq - in a rare step, noting that almost 2,000 troops had lost their lives - but she said the country needed to be clear about who they were fighting.
"This is not some grassroots coalition of national resistance, these are barbaric killers who want to provoke nothing less than a full-scale war among Muslims across the entire Middle East," she said.
"If we quit now, we will abandon Iraq's democrats at their time of greatest need. We will embolden every enemy of liberty and democracy across the Middle East. We will destroy any chance that the people of this region have of building a future of hope and opportunity. And we will make America more vulnerable.
"If we abandon future generations in the Middle East to despair and terror, we also condemn future generations in the United States to insecurity and fear...
"We have set out to help the people of the Middle East transform their societies. Now is not the time to falter or fade."
Ms Rice's comments come as Iraq prepares for a referendum on its proposed new constitution on 15 October.
In her speech, Ms Rice also defended the use of force, while making a veiled criticism of countries that did not back the invasion of Iraq.
"In a world where evil is still very real, democratic principles must be backed with power in all its forms: political and economic, cultural and moral, and yes, sometimes military," she said.
"Any champion of democracy who promotes principle without power can make no real difference in the lives of oppressed people."
She said Iraqis needed to maintain their commitment to building democracy and institutions, and that Iraq's neighbours must also provide support.