In a statement the group, which signs itself off as "Abu Hafs al-Masri Brigades/al-Qaeda" and which has been received with skepticism by some, boasts that its "death squad" had penetrated Spain, "one of the pillars of the crusader alliance".

The group singled out Britain, Japan, Italy and Arab country Yemen, which under US pressure has been cracking down on Islamic militants.

The claims could not be independently verified, and it is never certain that the claims and threats attributed to al-Qaeda really do come from the group.

Experts have said there are small groups that claim to speak in al-Qaeda's name, but that no central authority exists to speak on its behalf.

But while al-Qaeda leader Osama bin Laden and his chief lieutenant, Ayman al-Zawahiri, continue to elude the grip of the United States, their message of jihad (holy war) against America and its "crusader alliance" appears to have a formidable following.

In a series of e-mails and other statements to Arab media, bin Laden's network and its affiliates throughout the Arab world have claimed deadly attacks from Iraq to Istanbul, and warned that no US "collaborator" will be spared.

The Brigades of Abu Hafs al-Masri takes its name from Mohammed Atef, a top al-Qaeda member killed in the US-led military campaign on Afghanistan. It previously claimed the deadly bombings on the UN headquarters in Baghdad last August and those on two synagogues in Istanbul in November.

The Spanish press has stressed that Thursday's devastating attacks could have been the work of Islamic extremists rather than the armed Basque separatist group ETA, which the Spanish government initially blamed but now acknowledges may not have been the perpetrator.

US officials refrained from fingering Islamic militants just yet.

"We can't confirm the authenticity of the claim," said one official in Washington. "It's outside their normal mode of operandus."

Meanwhile, an audio tape with Quranic verses in Arabic and seven detonators were found in a van suspected of being linked to the bomb attacks in Madrid, Interior Minister Angel Acebes said.

The van, which had been reported stolen, was discovered in the east of the capital, Acebes told a media conference. The find added a new element to the investigation into the blasts.

Earlier Thursday, Acebes had said there was "no doubt" the Basque separatist group ETA was responsible for the attacks.

The UN Security Council Thursday unanimously passed a resolution condemning ETA as the perpetrator of the attacks, based on Spain's initial information.

But at his later media conference, Acebes said: "A new clue has been found and our duty and responsibility is to open a new line of investigation and to tell you this in all transparency because the citizens have the right to be informed."

The minister said: "The main lead remains ETA, but we must be very cautious and investigate other leads."

He said the verses in Arabic were those "usually used to teach the Quran," and that they "did not contain any threat".

Acebes left open the possibility that the tape might have been planted to mislead authorities.