"The Spanish state has continued its aggressions against Muslims in sending new troops to Iraq and announcing its intention to send new units to Afghanistan," the ABC newspaper quoted the Arabic language letter as saying.

It said it received the fax on Saturday signed by "Abu Dujana Al-Afgani (of the) Ansar al-Qaeda Europe group" which announced it was scrapping a "truce" designed to give Spain time to remove its forces.

"We are announcing the cancelling of the truce with effect from midday Sunday, April 4," the statement said, calling on Spain to desist from helping "enemies of the Muslim community -- the United States and its allies.

"If these demands are not met we will declare war on you and turn your country into a hell where blood will flow in rivers. This is our last warning."

Spain's incoming Socialist prime minister Jose Luis Rodriguez Zapatero has vowed to withdraw 1,300 Iraq-based troops by June 30 unless they come under UN command.

A relief force is, however, on its way to replace the current contingent.

The outgoing conservative government of Prime Minister Jose Maria Aznar deployed the troops last August in the face of massive popular disapproval.

However, since his election victory on March 14, three days after a series of train bombings killed 191 people in Madrid, Zapatero has indicated he would be willing to maintain a troop presence in Afghanistan, while sticking to his pledge to quit Iraq unless the UN takes control of operations there.

The fax described the March 11 bombings on four Madrid trains, the worst terror attacks in Europe since the 1988 bombing of a passenger jet over Lockerbie, Scotland, as "happy attacks".

The statement also claimed responsibility for a bomb discovered last Friday on a highspeed railway line between Madrid and Seville, saying it was "merely a warning" to show that "we can attack when and how we decide."