Arabic TV station al-Jazeera has broadcast an audio tape it says is by the al-Qaeda leader, Osama Bin Laden.
Opinion is divided as to whether the voice really is Bin Laden's
In it, the speaker says new attacks on the US are being planned, but offers a "long-term truce" to the Americans.
CIA analysts have concluded the voice on the tape was that of Bin Laden, making it the first time he has been heard from since December 2004.
However, other analysts familiar with Bin Laden's voice are divided as to whether the voice really is his.
The US quickly rejected the truce offer made on the tape.
"We do not negotiate with terrorists. We put them out of business," said White House spokesman Scott McClellan.
The speaker on the tape said the reason there had not been an attack in the US since 11 September 2001 was not because of superior US security, but because the group had been engaged in activities in Iraq - and because operations in the US "need preparations".
"The operations are happening in Baghdad and you will see them here at home the minute they are through (with preparations), with God's permission," he said.
US officials have said they believe Bin Laden hiding in a mountainous area on the border between Pakistan and Afghanistan.
There is no clear indication of when the tape was recorded.
Last month, al-Jazeera aired a videotape it said dated back to September, showing al-Qaeda's deputy leader Ayman al-Zawahiri.
In it, Zawahiri declared that, despite a prolonged absence and rumours about ill-health or possible injury, Bin Laden was alive.
Despite the warning of renewed attacks, the speaker also offered the US the chance of a long-term truce in light of the fact that US public opinion polls showed growing opposition to the war in Iraq.
"We have no objection to responding to this with a long-term truce based on fair conditions," the speaker said.
"We do not mind offering you a truce that is fair and long-term... so we can build Iraq and Afghanistan... there is no shame in this solution because it prevents wasting of billions of dollars.
"Your president is misinterpreting public opinion polls which show that the vast majority of you support the withdrawal of your forces from Iraq."
Bin Laden made Europe a similar truce offer following the Madrid train bombings of March 2004.
Correspondents say it is an attempt to frighten the public and drive a wedge between them and their governments, which say it is necessary to stay to distance in Iraq, not pull out troops.
The BBC's Justin Webb in Washington says that in the US the immediate political effect of the tape will probably be to boost support for President George W Bush.