The war in Iraq has transformed the al-Qaeda affiliate into a battle-hardened organization with piles of money, sophisticated recruiting networks and some of the world's most experienced and innovative bomb makers, officials and analysts say.

A US intelligence estimate released this week said al-Qaeda "core," the parent organisation led by Osama bin Laden, "will seek to leverage" those capabilities for attacks on the United States.

It offered no evidence that al-Qaeda in Iraq (AQI) is actively involved in such plots, however, and intelligence and military officials say most of its resources are currently tied down in Iraq.

But Ted Gistaro, the assessment's author, told reporters this week that al-Qaeda typically tries to tap into its affiliates' networks, recruitment pool and financial resources.

"Certainly when it comes to finances, we've seen al-Qaeda pull benefit from the relationship with AQI," he said. "And the concern is: what other parts of AQI might al-Qaeda core try to leverage or siphon off to bolster its own capabilities?"

Analyst say the concern raised in the assessment is new, and may reflect undisclosed intelligence of messages from al-Qaeda core urging AQI to carry out operations in Europe or the United States.

In late April, the Pentagon revealed it had captured a senior al-Qaeda operative, Abd al Hadi al-Iraqi, as he was trying to reach Iraq to manage al-Qaeda operations and possibly plot attacks in the west.

"al-Qaeda in Pakistan is clearly trying to inspire AQI to use its people and resources to mount attacks where they want," said a US counter-terrorism official, speaking on condition of anonymity.

AQI offers two things: money and an elaborate recruiting network, said Bruce Riedel, a former CIA officer now with the Saban Centre for Middle East Policy at the Brookings Institution.

"The al-Qaeda operation and the insurgency in Iraq is so popular in the Muslim world that there is pretty good evidence that AQ in Iraq is awash in money, that it has more than it can spend," he said.

Its recruiting networks "can also be reverse engineered so you could use them to train someone, send them back to their home, and then use that person for an operation against a target in Europe, in the Middle East, and conceivably, if they have the right passports, against the United States," he said.