The very fact sensitive national security information... was compromised is an indication of Fidel Castro's continuing desire to undermine the US Government

Senator Bob Graham

Ms Montes, a 45-year-old American citizen of Puerto Rican descent, faces up to 25 years in prison without the possibility of parole.

The case comes only a year after former FBI agent Robert Hanssen was arrested for spying for Russia.

The head of the Senate Committee on Intelligence, Senator Bob Graham, Ms Montes' case showed that Cuba was still a threat to the United States.

The 11 September attacks on New York and Washington heightened the need to "get her off the streets" and influenced the timing of her arrest, Roscoe Howard Jr, attorney for the District of Columbia said.

The four undercover agents whose identities she revealed are safe, he said.

Radioed instructions

Ms Montes was spying for Cuba from the time she started work at the DIA in 1985 - the same year Hanssen started selling US secrets to Russia - until her arrest on 21 September, prosecutors say.

Howard: Montes needed to be taken off the streets

The DIA, based at Bolling Air Force Base in Washington, provides analyses of foreign countries' military capabilities and troop strengths for Pentagon planners.

By the time of her arrest, she was a senior intelligence analyst and used short-wave radio and coded pager messages to give Cuba US secrets so sensitive they could not be fully described in court documents.

Montes received coded messages from Cuban intelligence by radio, which she then typed into a laptop computer equipped with a decryption programme and a string of numbers was decoded into Spanish text messages, according to court papers.

Ms Montes also reached her contact by calling a pager number from public telephones across Washington and leaving coded numeric messages saying "Message received" or "Danger", prosecutors said.

Ms Montes will be sentenced on 24 September.