An alleged member of an Islamist group that carried out the Barcelona van attack told a Spanish court today that the group had been planning a much bigger strike using explosives, a judicial source said.

The testimony to a closed hearing at Spain's High Court came from Mohamed Houli Chemlal, one of four detained suspects brought to Madrid to testify for the first time in court about the plot.

The accidental explosion [a day before] led them to change tactics, the police believe

Chemlal was arrested after being hurt in a blast at a house in Alcanar, southwest of Barcelona, a day before Thursday's van attack on the crowded Las Ramblas boulevard in Barcelona which killed 13 people.

Tuesday's court hearing was the first in a long legal process, and it could be months or even years before the case is brought to a full trial.

The four are the only alleged members of the group still alive after the driver of the van in Barcelona, 22-year-old Younes Abouyaaqoub, was shot and killed by police on Monday.

The van careered along Las Ramblas last Thursday, leaving a trail of 13 dead and 120 injured from 34 countries.

Islamic State claimed responsibility for the attack and a separate deadly assault, hours later, in the coastal resort of Cambrils, south of Barcelona.

In Cambrils, a car rammed passers-by and its occupants got out and tried to stab people. The five assailants, who were wearing what turned out to be fake explosive belts, were shot dead by police, while a Spanish woman died in the attack.

Police believe two other members of the cell died in Wednesday's explosion at the house in Alcanar, where police found 120 butane canisters they believed the group may have been planning to use for a big bomb attack.

The accidental explosion led them to change tactics, the police believe.

In little more than a year, Islamist militants have used vehicles as weapons to kill nearly 130 people in France, Germany, Britain, Sweden and Spain.