Suspected militant Abu Musab al-Zarqawi has purportedly written to Osama Bin Laden to admit his fighters face a struggle against US forces in Iraq.
Zarqawi is said to be behind many attacks against coalition troops
The letter published on the website of a radical Islamist group in Iraq, Ansar al-Islam, says the grip is tightening "on the holy warriors' necks".
It was not possible to check the statement's authenticity.
Zarqawi is the prime suspect in many attacks around the world, including the beheading of an American in Iraq.
The US says it is highly probable the 38-year-old Jordanian national killed American contractor Nicholas Berg last month.
US authorities also accuse Zarqawi, believed to be a close ally of Bin Laden, of masterminding a series of spectacular suicide bombings in Iraq.
The unsigned statement on Ansar al-Islam's web site has the title "The text of Zarqawi's message to Osama Bin Laden about the holy war in Iraq."
The letter says their fighters, drawn from the ranks of "the elite Sunnis", face four kinds of enemies in Iraq: the US forces, the Kurds, the Iraqi police, army and agents, and the Shias.
Iraqi police are in the militants' sights, the letter vows
It describes the Iraqi police and army as "the eyes of the occupiers, the ears with which they hear, and the hand with which they strike".
The holy warriors, the mujahideen, are determined to target them before the occupation takes hold, the letter vows.
But the fighters are facing difficulties and "with the spread of soldiers and police, the future is becoming frightening".
The aim is to undermine and abort the plans of the Iraqi government. Otherwise, the only choice is "to pack and leave for another land where we may carry the banner of [jihad] anew, or God may choose us as martyrs for His sake".
It was not possible to verify the letter's provenance. Some observers are sceptical, noting that al-Qaeda rarely admits weakness.
Correspondents also say the number of deadly bombings in Iraq in recent weeks suggest insurgents are not having too many difficulties in carrying out attacks.
Large sections of the letter were identical to one purportedly between Zarqawi and the al-Qaeda leadership which US intelligence said it intercepted earlier in the year.
A Cairo-based expert on Islamic militant groups, Dia'a Rashwan, told the Associated Press news agency that he believed the latest statement was merely a rehash of that message.
In it, Zarqawi also allegedly lamented the failure to expel US troops from Iraq and suggested fomenting sectarian violence between Sunnis and Shias to bolster the resistance.
US authorities have offered a $10m reward for information leading to his death or capture.
Last week, the US-led coalition in Iraq said one of Zarqawi's top aides, Umar Baziyani, had been arrested.