Police on Wednesday widened their probe into a terror cell that unleashed carnage in Spain last week, after a suspect admitted the jihadists were targeting monuments in Barcelona in an even bigger attack.

Officers carried out new raids overnight to root out a possible support network for the men, accused of ploughing vehicles into pedestrians on Barcelona's busy Las Ramblas boulevard and a seaside promenade in the resort town of Cambrils, claiming 15 lives including a young boy and wounding more than 120 people.

The international connections of the cell of mostly Moroccan nationals have also come under scrutiny, with talks later Wednesday between Spain and France's interior ministers expected to focus on the case.

The scale of the assaults being prepared by the jihadist suspects emerged during a preliminary court hearing Tuesday, when Mohamed Houli Chemlal, 21, told the judge the group was planning "an attack on an even greater scale, targeting monuments" using bombs.

- 'Nails, detonators, gas canisters' -

At least 500 litres of acetone, large quantities of nails and detonators as well as gas canisters have been found at a house in the town of Alcanar, court documents said.

They are ingredients of TATP -- the explosive of choice of the Islamic State group, which has claimed its "soldiers" carried out the attacks.

But an accidental blast at the bomb factory in Alcanar on August 16, the eve of the Barcelona van attack, forced the cell to alter its plans and turn to vehicles as killing machines.

- Another false start -

After losing their bombs and two cell members in the blast that was so powerful that "the mushroom cloud that was generated was visible several kilometres around", the jihadists put plan B into action.

They rented a van the next day, but again suffered a false start as it was involved in an accident not far from Cambrils.

A second van was used two hours later in the Barcelona rampage.

The suspects then bought four knives and an axe that were used to slash at pedestrians in Cambrils in the early hours of Friday.

After a full day court hearing, Chemlal and another suspect, Driss Oukabir, 28, were remanded in custody and charged with terror-related offences.

A third man, Mohamed Aallaa, 27, who owns the car used in the Cambrils attack, was granted conditional release, with the judge saying evidence against him was weak.

The judge gave himself three more days to decide if the fourth suspect, Salh El Karib should be remanded or released. His shop, which allows people to make calls abroad, was among targets of police raids late Tuesday.

An apartment in the village of Vilafranca del Penedes, 50 kilometres (30 miles) west of Barcelona, was also searched, a police spokesman told AFP.

The men in court Tuesday were the only surviving suspected members of the terror cell, which Spanish police claimed they had dismantled after gunning down the last man at large, Younes Abouyaaqoub, on Monday.

The 22-year-old Moroccan was the van driver in the Barcelona rampage which claimed 13 lives. While on the run, he also hijacked a vehicle and killed its driver.

- 'Imam wanted to blow himself up' -

Chemlal was injured in the Alcanar explosion that killed an imam, Abdelbaki Es Satty, who is thought to have radicalised him and other young suspects.

He only survived because he was out on the porch when the blast occurred.

He told the judge the imam had wanted to blow himself up. The judicial source said that among the four suspects, "two blamed the imam for the plot while another two denied knowing him".

In the rubble of the Alcanar house, police found a sheet of paper slipped into a green-coloured book, which said: "A brief letter from the soldiers of the Islamic State on the territory of Al Andalous to the crusaders, the sinners, the unjust and the corrupters".

Al Andalous is the name of the territories governed until 1492 by Muslims.

- Audi in Paris, imam in Belgium -

With the cell made up of mostly Moroccan nationals who lived in Spain, focus is turning to their links abroad.

Spanish press reported arrests in Morocco in relation to the case, but Rabat declined comment when contacted by AFP.

In Belgium, the mayor of the Vilvorde region told AFP that Satty spent time in the Brussels suburb of Machelen -- next to the city's airport -- between January and March 2016.

And French Interior Minister Gerard Collomb said the Audi used to kill people in Cambrils had been detected speeding in the Paris region while making "a very rapid return trip" days before the Spanish attacks.

At least one of the suspects also spent a night in Zurich in December, according to Swiss police.

source: AFP