US President George W Bush has used his speech on American Independence Day to warn that the nation was "still at war" with international terrorism.
Bush said US troops would remain actively engaged in the world
Speaking at Wright Patterson Air Force Base in Ohio, President Bush vowed to attack any "terrorist group or outlaw regime" that threatens America.
"We are on the offensive against terrorists... The United States will not stand by and wait for another attack or trust in the restraint and good intentions of evil men," he said.
President Bush was speaking as the Arabic satellite channel al-Jazeera broadcast what it says is a recent recording of former Iraqi President Saddam Hussein.
On the taped message - which al-Jazeera said was dated 14 June, the speaker said he was alive and well and living among Iraqis, and urged to fight the US-led occupying forces.
US officials said the Central Intelligence Agency would begin a detailed analysis of the tape to match it against known samples of Saddam Hussein's voice.
Flanked by fighter jets, attack aircraft and a Stealth bomber, President Bush reiterated his commitment to use force if he felt it was necessary.
"The enemies of America plot against us... We will act, whenever it is necessary, to protect the lives and the liberty of the American people," he said.
President Bush also promised that US troops would remain actively engaged in the world.
The BBC's Michael Buchanan says the president's patriotic, rally-the-troops speech made no mention of the situation in Iraq or the almost daily attacks on US troops there.
But our correspondent says that it comes at a time when more Americans are questioning the success of the US-led military campaign there.
The Americans have been facing armed resistance in Iraq, with at least 19 US troops killed in attacks since 1 May, when the president declared major combat operations over.
US troops are engaged in a series of large military operations in the area to root out supporters of the ousted president who have been staging increasingly bold attacks on American forces.
On Thursday, a sniper killed a US soldier guarding the Iraqi National Museum in Baghdad.
On Friday 18 soldiers were injured when mortar bombs were fired at one of the largest US military bases in Iraq, also near Baghdad - in Balad.
A few hours later US troops killed 11 Iraqis who ambushed their convoy north of Baghdad using rocket-propelled grenades and small arms fire, a US military spokesman has said.
Al-Jazeera said the tape purported to be Saddam Hussein was delivered to the station via telephone on Friday.
The voice urged Iraqis to resist occupying forces
The station's chief editor said that although they did not know the source, "we have no reason to doubt (the tape's) authenticity".
The BBC's Frank Gardner says the voice on the tape sounds like Saddam Hussein - but that even if it proves not to be him its psychological effects will harm the coalition's efforts.
He says the tape's message is designed to rally those loyal to the old Baath party, and convince Iraqis that Saddam Hussein will one day return.
The whereabouts of the former Iraqi leader himself have been a mystery since the fall of Baghdad on 9 April.
The broadcast comes as the top US official in Iraq, Paul Bremer, has announced a $25m reward for information leading to the capture of Saddam Hussein.