MADRID - A POWERFUL car bomb exploded on Thursday at a university in the northern Spanish city of Pamplona, wounding at least 15 people and setting a building on fire in an attack blamed on Basque separatists.

There was no claim of responsibility, but officials quickly pointed the finger at the militant Basque group ETA. Spanish police had arrested three suspected members of ETA on Tuesday in Pamplona and another in Valencia.

'ETA has once again displayed its vileness,' said Jose Antonio Alonso, spokesman in Parliament for Prime Minister Jose Luis Rodriguez Zapaptero's Socialist party.

The bomb detonated in a parking lot at the University of Navarra, shattering windows and setting other vehicles on fire, said Amaya Zaratiegui, spokeswoman for the university's clinic. Navarra is a northern region of which Pamplona is the capital.

Aparicio Caicedo, a 29-year-old Ecuadoran who is doing doctoral work at the university, said he was studying in the library, near the parking lot, when the bomb went off.

'Suddenly the whole building shook and there was a huge column of smoke. It was tremendous, a huge explosion,' Mr Caicedo told The Associated Press in a telephone interview.

He said the bomb went off at a time when many students and other people were walking by the targeted area.

Television footage showed a university building on fire at the ground-floor level and spewing thick black smoke.

The Spanish Interior Ministry office in Pamplona said 15 people were slightly wounded, apparently cut by flying glass.

Pamplona's mayor, Yolanda Barcina, told Cadena Ser radio she was surprised the casualty toll from the blast was not higher.

'We got lucky with the weather. It is raining, and there were fewer people than usual outside,' the station's website quoted her as saying.

ETA has killed more than 800 people since the late 1960s in its battle to create an independent Basque homeland straddling northern Spain and southwest France.

Navarra borders on the Basque region and is home to many Basque-speakers. ETA says it should be part of the independent homeland it wants to create.

ETA called a cease-fire in March 2006 but resumed attacks in December of that year after peace talks with Zapatero's government failed, setting off a car bomb at Madrid's airport that killed two people.

Since ending the cease-fire the group has been blamed for seven deaths, including three this year. ETA's last fatal attack was a car bombing in late September that killed an army officer in the northern town of Santona. -- AP