"According to the information we have, the suspects listed in the arrest warrants are believed to be so-called code names of CIA agents," the prosecutors said in a statement cited by Agence France Presse (AFP).

"The investigation will now focus on learning the actual names of the suspects."

The American spies are accused of abducting 43-year-old Khaled el-Masri in the Macedonian capital Skopje on 2003 New Year's Eve over allegations of having links to terrorist networks.

"These findings, as well as other information uncovered in the probe, led to the strong suspicion that these 13 identifiable people were involved in the abduction of el-Masri," said the prosecutors' office.

Though he was never charged with a crime, el-Masri was detained in a secret prison in Afghanistan where he was repeatedly tortured and abused.

After five months, the CIA released him saying they had the wrong man.

The German public broadcaster NDR said the 13 agents, most of them live in North Carolina, were facing charges of abduction and grievous bodily harm.

It added that Spanish authorities had learned the identities of all 13 agents and had copies of some of their passports.

Although all of the names were believed to be aliases, NDR said it was possible, using other data, to learn their real identities.

German arrest warrants are not valid in the US but if the suspects were to travel to the European Union they could be arrested.

The US Justice Department has declined to assist the Munich prosecutors.

The case raised tensions during US Secretary of State Condoleezza Rice's visit to Germany in December 2005.

After the meeting with Rice, German Chancellor Angela Merkel said Masri's case "was accepted as a mistake by the US government", although US officials later suggested her remark was the result of a misunderstanding.

Typical Rendition 

Masri, a car salesman and father of four, is pursuing a 75,000-dollar compensation claim against the CIA in US courts.

His case is a typical example of the infamous policy of extraordinary rendition.

An Italian court is deliberating whether to order the trial of 26 Americans and nine Italians implicated in the February 2003 abduction of Egyptian imam Hassan Mustafa Osama Nasr in Milan.

The CIA is also accused of collaborating with top Canadian officials in transferring Maher Arar, a Canadian of Syrian background, to his home country where he was tortured.

The extraordinary renditions program is one of the most controversial aspects of Bush's so called war on terrorism.

The CIA has long kept details of the program, which allows the transfer of suspects to third countries without court approval, a closely guarded secret.

Since 9/11, it has rendered more than 100 people from one country to another, usually with well-documented records of abuse, without legal proceedings.

Bush has strongly defended such transfers as "vital to the nation's defense."