British police have arrested a total of 13 men over the Manchester suicide bombing after two fresh raids Monday morning (AEST), as authorities warn that members of the terror cell could still be at large.

Greater Manchester Police said a 19-year-old man had become the latest suspect detained over terror attack, which left 22 dead and 116 injured when Salman Abedi detonated an improvised explosive outside an Ariana Grande concert six days ago.

The teenager was arrested during an armed swoop on an address in the Gorton area of Manchester.

A 25-year-old man was earlier held on suspicion of terror offences in the Old Trafford district.

Abedi’s father and younger brother were separately arrested in Libya over suspected links to Islamic State.

But British Home Secretary Amber Rudd warned that members of Abedi’s network are still potentially at large, as the terrorism threat level was lowered from ‘critical’ to ‘severe’ due to significant progress in the investigation.

Asked during an interview on BBC television whether some of the group were still at large, Ms Rudd said: “Potentially. It is an ongoing operation. There are 11 people in custody, the operation is still really at full tilt in a way.”

Ms Rudd said Abedi was known to British security services before the bombing but declined to comment on exactly what had been known about him.

But a security source has claimed the FBI tipped off MI5 in January this year with information that Abedi was part of a terror group, according to the Mail on Sunday.

The source claimed US agents had been monitoring Abedi since 2016 by intercepting his communications, and had been gathering information from Libya where his parents lived.

MI5 then investigated Abedi and other members of the terror cell, and believed the group was planning to assassinate a political figure, the source claimed.

“But nothing came of this investigation and, tragically, he slipped down the pecking order of targets,” the source told the Mail on Sunday.

Police have released a photo via CCTV footage of Salman Abedi the night of the bombing. Photo: Greater Manchester Police

Media have reported that people who knew Abedi had raised concerns about him and his views as long ago as five years before he carried out Monday’s attack.

“The intelligence services are still collecting information about him, but I wouldn’t rush to conclusions, as you seem to be, that they have somehow missed something,” Ms Rudd said.

British officials have confirmed he had recently returned from Libya and said police needed information about his movements from May 18 when he returned to Britain.

British police earlier released a photograph of Abedi, 22, on the night of the attack in Manchester and say they believe he assembled the device in an apartment in the city centre.

“We know one of the last places Abedi went was the city centre flat, and from there he left to make his way to the Manchester Arena,” Greater Manchester Police Chief Constable Ian Hopkins and Neil Basu, senior national coordinator UK Counter Terrorism Policing said in a joint statement.

#RunForManchester

Earlier Sunday (local time), tens of thousands of runners pounded the streets of Manchester in an annual fun run that became a symbol for unity and defiance.

Armed police patrolled among spectators at the Great Manchester Run, the large-scale event to take place since the bombing.

Attendees and spectators stood for a minute’s silence for the victims and casualties after giving emergency services a round of applause.

Runners then set off under the banner #RunForManchester wearing yellow ribbons and Manchester bee symbols, while “I heart Manchester” signs were dotted around the city.