An American woman in her 60s has been killed after being repeatedly stabbed in the back by a knifeman in a central London rampage as police rowed back on terrorism as a potential motive and said the attack was probably triggered by mental health issues.

The US citizen was knifed by the "large man" - who was reportedly wearing black shorts and a white t-shirt - as he silently and indiscriminately attacked pedestrians in Russell Square on Wednesday night.

She was helped by a family of Spanish tourists as she lay slumped against railings after being targeted by the knifeman, who witnesses said did not say a word as he also stabbed a man in his 40s in the chest and a woman in her 20s in the arm.

A 19-year-old man has been arrested on suspicion of attempted murder, having received treatment in hospital under armed guard after a Taser was discharged as he was detained by police shortly after 10.30pm.

The suspect is a Norwegian national of Somalian origin, but no evidence has been found of radicalisation, Scotland Yard's head of counter-terrorism Assistant Commissioner Mark Rowley said.

Police had earlier said they were keeping an open mind over the motive behind the attack - which took place in a popular central London tourist area close to the scene of the July 7 London bus bombing in 2005 - and said terrorism was "one line of inquiry that we should explore".