The CIA warned security bosses in Barcelona of a possible terror attack on Las Ramblas three months ago, according to a note published in a Spanish newspaper.

The note, published by El Periodico, is purportedly from a high-level US intelligence agency, although the identity of the sender is not revealed and its veracity has not been confirmed.

The document was apparently sent on May 25, and claimed the CIA had received "unsubstantiated information of unknown of unknown veracity" about an attack by Isis "against crowded tourist sites in Barcelona, Spain specifically, La Rambla Street" [sic].

El Periodico claims US intelligence agencies directly contacted Barcelona's Mossos d'Esquadra police force to pass on the warning., but it was ignored.

The newspaper also claims it published details of the CIA warning an hour before the attack on Las Ramblas took place on August 17.

However the head of Barcelona's security operation has rubbished reports of the warning, saying it is simply propaganda.

Joaquim Form, interior minister for Catalonia's regional government, said the reports were simply an attempt to undermine the Mossos d'Esquadra, Barcelona's local police force which played a key role in tracking down those responsible for the attacks.

Forn has said the document is a "hoax," adding "we don't have a direct relation with the CIA" because such a relationship is conducted through the national government in Madrid.

Sixteen people were killed in attacks on Las Ramblas and the nearby resort town of Cambrils. A number of people, allegedly part of a terror plot based in the Catalan town of Ripoll, have since been arrested while suspect Younes Abouyaaquob was shot dead by police.