Madrid, Spain - Spain has expelled two US embassy staff who managed to infiltrate the Spanish CNI intelligence service by bribing two of its officers, El Pais newspaper reported Thursday.

Government ministers had on Monday confirmed a report first published by the El Confidencial news website about the arrest of two Spanish intelligence officers on charges of passing on confidential information to Washington, without giving further details.

Suspicions first came to light after the two CNI agents were noticed accessing classified information that was not part of their remit, the paper said, though it remains unclear what information they passed on.

They were recruited by the two spies based at the US embassy who have since been kicked out of Spain, El Pais said.

"At least two US agents stationed at the US Embassy in Madrid, who were directly involved in the recruitment of CNI spies, have been discreetly expelled from Spain," the paper said.

Contacted by AFP, the defense ministry, to which the CNI reports, could not immediately be reached while the foreign ministry said it had "no comment" on the report.

The affair raised eyebrows as both Spain and the United States are NATO members that have a good relationship.

El Pais said both Defense Minister Margarita Robles and Foreign Minister Jose Manuel Albares had separately spoken to the US ambassador to ask for an explanation.

During an informal chat with foreign journalists on Tuesday, Prime Minister Pedro Sanchez acknowledged that the case had caused unease in Madrid but steered clear of criticizing Washington and insisted the situation had been resolved.

El Pais said the two agents were arrested "two months ago" following an investigation that began before the summer, describing one as a "middle-ranking member of CNI who had been recruited... to obtain secret information on demand in exchange for a large sum of money."

It said the other agent was his assistant.

The US administration has not commented on the affair.