Spain is investigating a police report which claimed US Central Intelligence Agency (CIA) planes carrying terrorist suspects had made secret stopovers on Spanish soil, a Spanish minister said on Tuesday.

Spanish Interior Minister Jose Antonio Alonso told Teleconco television that an earlier report in Spanish daily El Pais, if confirmed, could hurt relations between Madrid and Washington.

Using Spanish airports for secret flights to transport terrorist suspects was "a serious and intolerable issue" which could affect "the level of relations between the two governments," said the minister.

Spanish daily El Pais quoted a police report as saying that CIA planes carrying alleged extremists made at least 10 secret stopovers in the Balearic Islands off Spain's Mediterranean coast.

The National Intelligence Center, the main branch of the Spanish secret service, had asked the CIA not to use Spanish airports for ferrying prisoners, El Pais said, quoting unidentified government sources.

The US embassy in Madrid has refused to comment on what it termed as "a sensitive issue."

Source: Xinhua