"The enemy ... has mobilised most of its assets and potential to destroy Islam in Falluja," said the tape attributed to Abu Musab al-Zarqawi.
"Once they have finished in Falluja, they will head towards you," the voice on the tape said on Monday, urging fighters to seize the initiative.
"Be cautious and foil their plan," he said, in implicit recognition that the week-long battle in Falluja, the besieged city west of Baghdad, was virtually lost for the fighters under US assault for the past week.
The message posted on a website was addressed to "the heroes of Baghdad and al-Anbar", the province which is home to Falluja, to "the lions of Mosul" in northern Iraq and the "lion cubs of Diyala, Samarra and Salahadin".
"The enemy are avoiding fighting us for fear of being dispersed and of a war of attrition," and "they are weak and cannot widen the battle", it said, warning that US forces would confront one fighter stronghold after another.
Prolong the battle
"Don't be satisfied with repelling them from your territory... Shower them with rockets and mortar rounds," said the man claiming to be al-Zarqawi, apparently giving orders to his fighters.
Al-Zarqawi implicitly recognised
that Falluja had fallen
"Cut all their supply lines, the main and secondary ones and carry out ambushes on these routes," he said.
The authenticity of the tape was impossible to verify.
"Make sure that the initiative in the battle remains in your hands. Prolong the battle because each day, for them, is worse than the one before," said the voice.
"The enemy is counting on the time factor to reduce the rhythm of the battle and lessen its intensity," he said.
In a similar message posted on the internet on 12 November, a voice attributed to al-Zarqawi, who has a $25-million US bounty on his head, called on his fighters to resist.