A Spanish judge has jailed one of three suspected al-Qaeda members arrested for allegedly planning an attack in Europe, postponing a decision on the other two until Sunday.

The judge on Friday jailed a Turk accused of being an al-Qaeda operative for "possession of explosive substances and devices with terrorist aims", but did not agree with prosecutors that the man belonged to a terrorist group, the source said.

Two Chechen suspects also accused of belonging to al-Qaeda are currently being held for questioning while police continue their investigation.

The Turkish man's lawyer said his client, who denied all charges against him, is an engineer for a multinational company who has lived in Gibraltar for seven years and has legal residence in Britain.

After the men's arrest on Wednesday, Interior Minister Jorge Fernandez Diaz said investigators had a "clear indication" that the men had been planning an attack in Spain or elsewhere in Europe.

He called the suspects "extremely dangerous people", including one who was "a very important operative in al-Qaeda's international structure".

Two of the suspects were travelling in a bus towards France when they were arrested in a layby at Valdepenas by a police special operations group, according Spain's El Pais newspaper.

The Turkish man was reportedly arrested in a flat near La Linea de la Concepion, in the southern province of Andalucia, where police also discovered explosives allegedly being stored by the group.

In attacks on the capital, Madrid's, train system eight years ago, 191 people died and 1,800 were wounded.

Source: Agencies