One of most-wanted members of the Basque separatist group ETA has been arrested in southern France, the Spanish Interior Ministry announced this week.

A statement issued by the Spanish government said Inaki Imaz Munduate, also known by the nom de guerre Xabi, was arrested Monday at an apartment in French border town of Hendaye, five years after he fled Spain's Basque country.

Imaz had been hiding in Ireland with false documentation provided by ETA until a few months ago -- part of an alleged strategy designed by the terrorist group to relocate a part of its leadership to avoid a crackdown by French police, Madrid said.

Imaz, 33, is alleged to be a key member of ETA's San Sebastian command unit and a founder of its political wing.

He was arrested on a warrant issued by the Spanish National High Court "for an offense relating to collaboration with an armed gang" by members of the French Judicial Police in collaboration with the Spanish National Police Force.

Spanish officials said Imaz was considered a "legal" member of the San Sebastian command unit, whose job included offering his flat in the Basque country capital to Jose Angel Lerin Sanchez, also known as Jaxinto, an "illegal" ETA activist and identified as its alleged leader.

Authorities said they found "various weapons and ammunition" in the Hendaye apartment, as well as hand-written documents allegedly written by Imaz laying plans for future terrorist attacks.

The documents allegedly included an analysis of the schedules followed by police at their barracks in San Sebastian and the address of the parents of a Basque Police Force officer.

Imaz also allegedly had information on the movements of local politicians from the Basque town of Pasajes de San Juan in Guipuzcoa province.

The Interior Ministry alleged Imaz was part of the same ETA unit as Ignacio Lerin Sanchez, nicknamed Belea, who was arrested in London along with another suspected ETA member in June.

Sanchez, 38, and Antonio Troitino, or Anxton, 55, were arrested during a June 29 raid of a west London home, where the men had been living with fake identities, The Daily Telegraph reported.

Troitino in 1987 was sentenced to more than 2,000 years in prison for the murders of 22 people, but Spanish penal law at the time limited the maximum jail term to 30 years, and he was released in April 2011 after serving 24 years based in part on good behavior.

His release sparked outrage, and the National Court revoked his release within days and ordered his re-arrest but he had gone into hiding.

"These arrests have dealt a powerful blow to ETA. Troitino was a key figure in the terrorist organization and had direct links to the current leadership," the Interior ministry said in June.

The ETA is blamed for more than 820 deaths and is considered a terrorist organization by Spanish, French, British and U.S. authorities as well as the European Union.

The group, which has campaigned five decades for an independent Basque homeland in parts of southwestern France and northeastern Spain, declared "a definitive end to its armed struggle" in October. ETA has been calling for negotiations with Spanish and French authorities.

Eighteen members of the terrorist gang ETA have been arrested this year, Madrid said.