A seven-year-old Sydney boy is missing and his mother is in a serious condition in a Barcelona hospital after being caught up in the deadly terror attack.

Key points: Jom Cadman and her son Julian were in Barcelona for niece's wedding

Jom Cadman and her son Julian were in Barcelona for niece's wedding Boy's father said he had spoken to pair only hours before the attack

Boy's father said he had spoken to pair only hours before the attack Earlier Julie Bishop said one Australian was "unaccounted for", four injured

Julian Cadman was separated from his mother Jom Cadman during the van attack on the Las Ramblas tourist boulevard, the family told the ABC.

The boy's father, Andrew Cadman, said he had spoken to the pair only several hours before the attack.

Mr Cadman was flying to Barcelona on Friday night to search for his son.

Julian Cadman's family is sharing his photo in a bid to find him ( Supplied )

Family members have been sharing the photo online in a bid to find Julian.

"If you know anyone in that area that you can share this with, please do so," the boy's godfather Colin Baxter wrote on Facebook.

Ms Cadman and her son had travelled to Barcelona to attend her niece's wedding.

Prime Minister Malcolm Turnbull suggested Australians take a moment to pray for the young boy, adding that his thoughts were with the family as they waited for answers.

"I think we should all in our quiet moments say a prayer for that little boy. All of us as parents know the anguish his father and his whole family is going through as they rush to seek to find him in Barcelona," Mr Turnbull said.

Sorry, this video has expired Turnbull asks for prayers for missing seven-year-old in Barcelona

Australians among 130 injured in van attack

Earlier, Foreign Minister Julie Bishop said an Australian remained "unaccounted for" after the van attack on the tourist strip which left 13 dead.

A 14th person died after being injured in a related attack in the seaside town of Cambrils.

"We condemn in the strongest possible terms these brutal and cowardly attacks, and in the case of Las Ramblas clearly designed to harm and affect tourists who were visiting the area," she said.

Witnesses to the van attack said the white vehicle had zigzagged at high speed down Las Ramblas, ramming pedestrians and cyclists, sending some hurtling through the air and leaving bodies strewn in its wake.

More than 130 people were injured in the attacks in Barcelona and Cambrils, and authorities have warned the death toll could rise.

Ms Bishop said four Australians were injured in the attack.

Sorry, this video has expired Australian unaccounted for following Barcelona terrorist attack

Spanish police arrested two people after the attack, but the driver fled on foot and remained at large on Friday morning (local time).

Police said the attack in Barcelona was connected to two other incidents: one in the resort town of Cambrils, south of Barcelona, and one in the town of Alcanar.

Five suspected terrorists who police said were carrying bomb belts were shot dead in Cambrils after their car was reportedly driven into pedestrians, injuring seven people — one critically.

An explosion in a house in Alcanar, where police believe a homemade bomb was being prepared, killed one person and injured another.

Ms Bishop said there were no reports that any Australians were involved in the other incidents.

A floral tribute left on the pavement on a street in Las Ramblas in Barcelona. ( AP: Manu Fernandez )

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