The four were among 16 arrested in January in the northeast region of Catalonia and subsequently released.

A High Court judge on Friday summoned the men to testify after an FBI report said the substances found in their home could have been mixed with petroleum products to produce "homemade napalm," which can be used in incendiary bombs.

The men, who had been freed on bail, are charged with cooperation with an armed group.

Algerian group

No date has been set for their court appearance, although sources said it was likely to be within the next few weeks.

They had been accused by the Spanish government of belonging to Algeria's Salafist movement, a splinter group of the Armed Islamic Group with alleged links to al-Qaida.

Separately, the Interior Ministry said Spanish police had arrested Youb Saoudi, one of the 16 originally detained in January, at the request of Algeria.

'Terrorist network'

Algeria wants to extradite Saoudi, an Algerian citizen, to face charges of setting up "an armed terrorist group", the ministry said in a statement.

It was not clear if he was also one of the four men summoned by the judge, because officials did not release their names.

Spanish Prime Minister Jose Maria Aznar said at the time of January's arrests his government had smashed a "major terrorist network".

Authorities later released all but two of the men in March after suspected chemical weapons material found at their homes turned out to be laundry soap.

September 11

Spain, one of the staunchest supporters of the US-led war in Iraq, has made dozens of high-profile arrests since the September 11 attacks.

But many of those accused of links with groups such as al-Qaida have since been released.

On Thursday, another Spanish judge ordered a journalist from  Aljazeera television networkto remain in jail without bail and stand trial on charges of belonging to an al-Qaida cell in Spain.