Family and friends of a seven-year-old Australian boy missing after the Barcelona terror attack are pleading with people worldwide on social media to help find him.

Julian Cadman became separated from his seriously injured mother Jom in the terrorist attack on Friday.

The mother and son from Sydney were in Spain for a wedding this weekend.

Ms Cadman's father-in-law, Tony Cadman, says she is in a serious but stable condition in hospital, but his grandson is missing.

"Julian is seven-years-old and was out with Jom when they were separated, due to the recent terrorist activity," he said in a Facebook post.
British Prime Minister Theresa May said the UK is urgently looking into reports a child with dual nationality, believed to be Julian, is missing, the BBC reports.

Meanwhile, the driver of the van that ploughed into crowds in Barcelona, killing 13 people and injuring scores of others, was one of five men shot by police in the Catalan seaside resort of Cambrils hours later, Spain's El Pais and El Periodico newspapers are reporting.

Catalan police had said earlier that it was possible that the driver was among those killed in the second incident, but they were not certain.

The driver fled after speeding along a pedestrianised section of Las Ramblas, the most famous boulevard in Barcelona, leaving a trail of dead and injured.

Investigators on Friday were also trying to pin down links between the Barcelona attackers and a group of five assailants, armed with an axe and knives and wearing fake explosive belts, who were shot dead by police hours later in a seaside resort further down the Mediterranean coast.