The 19-year-old suspect arrested after a fatal knife attack in central London does not appear to have been motivated by terrorism, police said following initial investigations on Thursday.

"So far we have found no evidence of radicalization or anything that would suggest the man in our custody was motivated by terrorism," said Mark Rowley, assistant commissioner of the Metropolitan Police.

"The suspect is a Norwegian national of Somali ancestry," Rowley said. "I stress, though, that all the [investigation] work we have done does not suggest those [details] are relevant to the motivation for his actions."

The woman murdered in the attack late Wednesday is a US citizen, he said. She is believed to be in her 60s and was treated at the scene near Russell Square, close to the British Museum in central London, but pronounced dead soon after.

The five people injured include Australian, US, Israeli and British citizens, Rowley added.

The two women and three men sustained "various injuries," police said.

Norwegian police confirmed that the suspect is a Norwegian citizen who was "registered as having emigrated from Norway in 2002."

Police said earlier that the suspect was transferred to a London police station on Thursday morning after he was treated in hospital for unspecified injuries.

Armed officers detained the suspect after they discharged a taser at him around 10:30 pm (2130 GMT) on Wednesday.

Rowley had said earlier Thursday that "mental health was a factor in this horrific attack."

London Mayor Sadiq Khan called on residents to remain calm after the stabbings.

"I urge all Londoners to remain calm and vigilant ... We all have a vital role to play as eyes and ears for our police and security services," Khan said.

The attack came hours after the deployment began of 600 extra armed police officers on the streets of London to counter the threat of terrorism.

"As a precautionary measure, Londoners will wake up this morning to notice an increased presence on the streets of officers, including armed officers, today," Rowley said in a statement earlier Thursday.

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