LEON, Spain (AP) _ A bomb believed set by Basque extremists severed a train track in northern Spain today, the fourth such blast in a week-old terror campaign against the state railway, a company official said.

The radical separatist group ETA said Aug. 13 it would set bombs along the Madrid-Valladolid-Leon-Gijon line and a bomb exploded along the track the following day. Other blasts damaged rails Friday and Sunday.

ETA has not claimed responsibility for the blasts, which caused no injuries.

The explosion occurred 30 miles north of Leon. The line was open at the time of the blast, although no trains were nearby, the official said. She said service between Leon and Gijon was diverted to an alternate route.

The ETA terror campaign was seen as an attempt to damage Spain's tourism industry. About 50 million tourists visited the country last year.

ETA, which stands for Homeland and Liberty in the Basque language, has claimed responsibility for the deaths of more than 600 people since 1968. Most of the victims have been police or military personnel.