Osama Bin Laden's deputy has said the defeat of US-led forces in Iraq and Afghanistan is only a matter of time.
Zawahri: "The Americans are hiding in their trenches"
In a video aired on Arabic TV station al-Jazeera, Ayman al-Zawahri said the Americans were "between two fires".
"If they continue they will bleed to death, and if they withdraw they lose everything," he said.
Al-Jazeera reported that he talked about the situation in Sudan's Darfur region, indicating that the recording was recent.
Zawahri, a bespectacled former Egyptian surgeon, is believed to be Bin Laden's spiritual adviser.
He is believed to be hiding with al-Qaeda leader Bin Laden on the Afghan-Pakistan border.
Al-Jazeera said it had received the tape exclusively, without giving any details.
The tape was aired two days before the third anniversary of the 11 September attacks on the US, which were blamed on al-Qaeda.
The attacks led to the US invasion of Afghanistan aimed at destroying al-Qaeda camps and the Taleban regime that hosted them.
The tape was also released just hours after Pakistani jets bombed a training camp on the Afghan border, believed to be used by foreign militants.
The Pakistani military said more than 50 people were killed in the camp in South Waziristan. They said most of the dead were Chechen, Uzbek and Arab militants with suspected al-Qaeda and Taleban links.
In the video message, Zawahri was wearing a white turban and had a machine gun at his side.
According to al-Jazeera, he said the era of security for the US had gone, and Americans would not enjoy it again if their government did not stop what he called its crimes against Muslims in Afghanistan, Iraq, and Palestine.
The TV channel added that Zawahri claimed there was a plan to tear apart the Arab and Islamic worlds, including the Arabian Peninsula, Egypt and Sudan.
In comments broadcast by the channel, he said US troops were lying low in east and south Afghanistan, which had become an "open arena for the mujahideen".
"The Americans are hiding in their trenches and refuse to come out to face the mujahideen, as the mujahideen shell them, fire on them and cut roads off around them."
Zawahri said their only defence was aerial bombing, which succeeded only in wasting American money.
"As for Muslim Iraq," he added, "the mujahideen in it has turned America's plan upside down after the interim government's weakness became clear.
"America's defeat in Iraq and Afghanistan has become a matter of time, God willing."
Sentenced to death
Zawahri is believed to be the architect of the al-Qaeda ideology.
In 1998, he was the second of five signatories to Bin Laden's notorious "fatwa" calling for attacks against US civilians.
He was a key figure in the Egyptian Islamic Jihad group, which later merged with al-Qaeda.
Zawahri's wife and children were reported killed in a US air strike in late November or early December 2001.
He has been indicted in the US for his alleged role in the 1998 bombings of US embassies in East Africa, and was sentenced to death in Egypt in absentia for his activities with the Islamic Jihad group in the 1990s.