British police arrested two more people on Thursday in a hunt for accomplices of two British men of Nigerian descent accused of hacking a soldier to death on a London street in revenge for wars in Muslim countries.

The two suspected killers, now under guard in hospitals, had been known to security services before Wednesday's daylight attack, security sources said.

Another man and a woman, both aged 29, were detained on suspicion of conspiracy to murder.

One of the assailants, filmed calmly justifying the killing as he stood by the body holding a knife and meat cleaver in bloodied hands, was named by acquaintances as 28-year-old Londoner Michael Adebolajo - a British-born convert to Islam.

Adebolajo and the other man, who may have been born abroad and later naturalised as British, were shot by police at the scene. Officers on the case raided six homes on Thursday.

Prime Minister David Cameron held an emergency meeting of his intelligence chiefs to assess the response to what he called a "terrorist" attack; it was the first deadly strike in mainland Britain since local Islamists killed dozens in London in 2005.

"We will never give in to terror or terrorism in any of its forms," Cameron said outside his Downing Street office.

"This was not just an attack on Britain and on the British way of life, it was also a betrayal of Islam and of the Muslim communities who give so much to our country."

U.S. President Barack Obama condemned it "in the strongest terms", adding in a statement: "The United States stands resolute with the United Kingdom, our ally and friend, against violent extremism and terror."

A source close to the investigation told Reuters that both attackers were known to Britain's MI5 internal security service.

Adebolajo had handed out radical Islamist pamphlets but neither was considered a serious threat, a government source said.

Another source close to the inquiry said the local backgrounds of the suspects in a multicultural metropolis - nearly 40 percent of Londoners were born abroad - and the simplicity of the attack made prevention difficult:

"Apart from being horribly barbaric, this was relatively straightforward to carry out," the source said. "This was quite low-tech and that is frankly pretty challenging."

Anjem Choudary, one of Britain's most recognised Islamist clerics, told Reuters Adebolajo, was known to fellow Muslims as Mujahid - a name meaning "fighter": "He used to attend a few demonstrations and activities that we used to have in the past."

He added that he had not seen him for about two years: "He was peaceful, unassuming and I don't think there's any reason to think he would do anything violent," Choudary said.

A man called Paul Leech said on Twitter he was at school in the suburb of Romford with the man seen claiming the attack: "Michael Adebolajo u [sic] make me sick," he wrote. "How could someone who was a laugh and nice bloke at school turn out like that."

British investigators are looking at information that at least one of the suspects may have had an interest in joining Somalia-based Islamist rebel group al Shabaab which is allied to al Qaeda, a source with knowledge of the matter said.

Al Shabaab linked the attack to the Boston bombing and last year's gun attacks in the southern French city of Toulouse: "Toulouse, Boston, Woolwich ... Where next? You just have to grin and bear it, it's inevitable. A case of the chickens coming home to roost!" the rebels said on Twitter.