London's Metropolitan Police have confirmed a woman who was killed in a knife attack in London was an American citizen.

Police say the attack suspect is a Norwegian national of Somali origin.

They say the attack was likely triggered by mental health issues, and there is no evidence that terrorism was the motive.

Police were called to reports of an armed man attacking people in Russell Square at 10.33pm on Wednesday.

A 19-year-old was arrested on suspicion of murder six minutes later at 10.39pm after a Taser was fired by one of the officers. He is now under police guard in hospital.

An unnamed woman, aged in her 60s, was treated at the scene but was pronounced dead a short time later.

The other victims - two women and three men - are said to have suffered "various injuries". Two remain in hospital but their condition is not known.

As a precautionary measure Mr Rowley said extra police would be deployed in London on Thursday to provide reassurance.

A cyclist passing Russell Square shortly after the stabbing was flagged down by a Spanish family helping the victims.

Fernando, from Brazil, saw a woman in her 60s with a stab wound to her back.

He said: "She had her head in a Spanish woman's lap, the lady, who was trying to keep her alive by talking to her."

"Wall to wall with armed police"

Fernando also saw an Englishman aged about 45 with an injury to his side and a woman in her 20s with a wound on her upper arm.

"The lady from the Spanish family told me that the guy had passed by, running, stabbing people," he said.

"They said he had been wearing black shorts and a white t-shirt and was chubby. They said they thought he was white but he was covering his face like this (with his arm)."

"Apparently he wasn't shouting anything."

Another witness, Zuhair Awartani, gave a conflicting description of the attacker, saying he saw a "dark-skinned man" being arrested.

Pol O'Geibheannigh was walking back from the cinema when he saw a woman lying in a pool of blood.

He said: "It was just wall to wall with armed police. You could see a woman on the floor. She was lying in a pool of blood with a red blanket on top of her. I went into the bar and it wasn't until an hour later that they put the tent up."

As morning broke officials were seen removing a body bag from the area.

Mayor of London Sadiq Khan urged calm, saying: "I have spoken with the Commissioner, and the Assistant Commissioner, who have assured me that our police officers are working extremely hard to investigate exactly what has happened and to keep the rest of us safe."

"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 attack came on the day London police announced it was to increase the number of armed patrols in London as part of anti-terrorism plans.