French police have arrested Alejandro Zobarán Arriola, ''Xarlas'', believed to be the military chief of the armed Basque group ETA, antiterrorist sources said on Thursday.

Three other presumed members of ETA were also arrested in an operation north of France near the Belgian border, according to antiterrorist sources.

The arrests took place in a house in the town of Boulancourt in the Pas-de-Calais region, where at least two firearms and documents were seized.

According to the same sources, the operation was continuing during the night of Thursday to Friday and there might be more arrests.

This police operation follows other arrests of presumed ETA members in the last two weeks. Daniel Pastor, Íñigo Zapirain, Beatriz Etxebarria and Lorena López, believed to be wanted for attacks including the killing of anti-terrorism officer Eduardo Antonio Puelles in 2009 and the car-bombing of a Civil Guard barracks in the northern city of Burgos the same year, were arrested last week in Biscay.

French police also arrested last week in Bayonne Gregorio Jimenez-Morales, a senior member of the armed group, who spent at least 10 years in prison for violent acts associated with ETA.