British police say a man who stabbed six people in London's Russell Square was a Norwegian of Somali origin, but they have found no signs of "radicalisation".

Assistant Commissioner Mark Rowley said on Thursday the investigation "increasingly points to mental health issues" as the possible cause of the attack.

"We believe this was a spontaneous attack and that the victims were selected at random," he said.

"So far we have found no evidence of radicalisation that would suggest that the man in our custody is in any way motivated by terrorism."

The 19-year-old man was arrested on suspicion of murder following Wednesday's attack in the city centre, which was cordoned off by police.

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Russell Square is a busy tourist area with a string of high-end hotels and is also close to the British Museum and the University of London.

Rowley said one person was killed in the attack - an American woman.

The five injured people are British, American, Israeli and Australian. None has life-threatening injuries.

In an earlier statement, Rowley had said "possible terrorism links" were also being investigated, but the line was later retracted from the second statement.

The American victim, believed to be in her 60s, was treated by paramedics at the scene but pronounced dead.

"Two people who were injured in the Russell Square incident remain in hospital. Three others have been discharged," police said on Twitter.

The arrested man is in police custody after initially receiving treatment at hospital.

Police were called to Russell Square at 10:33pm local time (21:33 GMT) on Wednesday following reports a man armed with a knife was attacking people. 

The man was arrested six minutes later, with one of the officers firing a Taser electroshock gun.

Mayor's assurance

In a statement on Thursday, London Mayor Sadiq Khan said: "Safety of Londoners is my number-one priority.

"I urge all Londoners to remain calm and vigilant. Please report anything suspicious to the police.

"We all have a vital role to play as eyes and ears for our police and security services and in helping to ensure London is protected."

The threat level in Britain remains at "severe", its second highest level, meaning a strike is "highly likely".

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London police had already promised to deploy more armed offices after a spate of deadly attacks in other European countries.

Police chiefs and security bosses have repeatedly warned that fighters supporting the Islamic State of Iraq and Levant (ISIL also known as ISIS) group want to carry out attacks against Britain.

London police said on Wednesday they would deploy an additional 600 armed officers across the capital to protect against the threat of attacks.

Rowley has previously warned that ISIL was seeking to radicalise vulnerable people with mental health issues to carry out attacks.

He said that, in some operations, police commanders have taken advice from specialist psychologists.

A 19-year-old man was arrested in Russell Square after the attack as police swarmed around the area [Will Oliver/EPA]

London was hit by coordinated attacks on July 7, 2005, when four suicide bombers targeted three underground trains and one bus, killing 52 people.

One of the bombs was detonated on an underground train travelling between King's Cross and Russell Square.

Since then, dozens of plots have been foiled and there have been smaller-scale attacks, such as the killing of an off-duty soldier on a street in southeast London by two people in May 2013.

Source: Agencies