France raised its terror alert late Thursday, posting more soldiers on public transport, and also stepped up security along its border with Spain in response to the train bombings in Madrid.

Officials in Spain blamed the rush-hour bombings on the Basque separatist group ETA (search) but said they have not ruled out Islamic radicals. The Egyptian newspaper Al-Quds al-Arabi said it had received a claim of responsibility issued in the name of Al Qaeda (search).

France raised its color-coded terror alert from yellow to orange, the second level in the four-level system. Prime Minister Jean-Pierre Raffarin's office said more soldiers would patrol trains and subways.

At checkpoints along the Spanish border, long lines formed in the first few hours after the attacks as police searched cars and trucks. The situation had returned to normal by early afternoon.

The French Basque region has long been a haven for militant Spanish Basques, although it has largely been spared the violence that has scarred the Spanish Basque provinces, just across the border.

A top French anti-terrorism official said a "small door" must be left open for the possibility that Muslim extremists were behind the carnage. Spain has been a strong ally of the United States in Iraq and there have been numerous arrests of alleged Islamic radicals in Spain.

The Madrid attacks — 10 explosions in rush-hour trains — and the absence of any warning does not correspond with the usual ETA profile, said Gilles Leclair, director of France's Anti-Terrorism Coordination Unit.

President Jacques Chirac (search) condemned the Madrid attacks and pledged "solidarity with Spain in fighting against this abominable scourge."

"Nothing ever justifies barbarity. Democracies must be — and will be — united in combatting this without weakness," Chirac said.

As well as a haven, ETA is suspected of using France as a conduit for weapons. It allegedly joined up with a tiny separatist movement in Brittany in western France to steal eight tons of dynamite from a warehouse in 1999. Some of the explosives are thought to have been used in attacks by ETA and the Breton group.

In recent years Spain and France have stepped up cooperation in their fight against separatists. More than 130 Basque militants are being held in French prisons — half of them convicted of terror-related crimes and the rest awaiting trial, according to judicial officials.