REPUBLIKA.CO.ID, 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.

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 is flying to Barcelona tonight to search for his son.

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.

Earlier today, 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.

Also read: Indonesia condemns terror attack in Barcelona

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 100 people were injured in the attack and authorities have warned the death toll could rise.

Ms Bishop said four Australians were injured in the 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.

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