The US has put a $25m bounty on Zawahiri's head

US intelligence published the letter in full, saying it was intended for the alleged head of the movement in Iraq.

In it, Ayman Zawahiri appears to question insurgents' tactics in attacking Shia Muslims in Iraq.

The claims by US intelligence are "based on imagination", a statement posted on an Islamist website.

"We in al-Qaeda organisation announce that there is no truth to these claims, which are only based on the imagination of the politicians of the Black [White] House and their slaves," the statement said.

It is not possible to verify either the letter or the subsequent denial.

News of the 6,000-word document first emerged last week. US officials insisted they believed it was genuine and recent.

Cryptic reference

According to US intelligence officials, the letter offers a remarkable insight into al-Qaeda thinking.

Many of your Muslim admirers amongst the common folk are wondering about your attacks on the Shia. The sharpness of this questioning increases when the attacks are on one of their mosques... My opinion is that this matter won't be acceptable to the Muslim populace however much you have tried to explain it

Letter US says is from Zawahiri to Zarqawi Zawahiri letter in full (58K) Most computers will open PDF documents automatically, but you may need to download Adobe Acrobat Reader. Download the reader here

The Americans will not say exactly when or how they intercepted it, except that it was during operations in Iraq.

The letter is not directly addressed to Zarqawi - although there is a cryptic reference to him.

But US officials say it is clear he is the intended recipient. It certainly heaps praise on whoever it was meant for.

But much of its tone also appears to be cautionary, as if trying to rein in events in Iraq, and suggesting possible tensions within al-Qaeda - "don't let your eyes lose sight of the target", it says.

'Four goals'

It sets out a detailed list of four incremental goals - expel the Americans from Iraq, set up an Islamic emirate in Iraq, extend the jihad "to the secular countries neighbouring Iraq", and "the clash with Israel".

But, crucially, it says the strategy requires popular support.

And it hints that some of the more brutal tactics employed by insurgents may be counterproductive.

The letter also suggests that the withdrawal of the US from Iraq could come quickly - as happened in the Vietnam War.

BBC world affairs correspondent Nick Childs says, whether this is all genuine, whether Zarqawi ever received the letter, and what impact it might have are not clear.

What is clear is that the level of insurgent violence is putting enormous political pressure on the Iraqi government and the US-led coalition at the moment.

So the fact that this is brought to light now may not be coincidental, our correspondent says.

Finally, the letter hints at the difficulties for al-Qaeda's top leaders, saying the operations of the Pakistani army in that country's tribal areas are "the real danger".