Norway has warned of a "concrete terror threat" from people with links to Syrian fighters, with an attack possible "within days".

Benedicte Bjoernland, the head of Norwegian security service PST, said her agency had received "reliable information" from foreign officials.

"It was unspecific about what the target might be," she said, adding there was no information about how or when an attack might take place.

The agency said officials would work closely with counter-terrorism teams to verify the information and establish whether any extra security is needed.

About 50 people have travelled from Norway to fight in Syria, around half of whom have now returned, according to PST.