Members of Manchester suicide bomber Salman Abedi's network are still potentially at large, British interior minister Amber Rudd said on Sunday, after the terrorism threat level was lowered due to significant progress in the investigation.

Police have said they have arrested a large part of the network behind the bombing, which killed 22 people at a concert hall, and two more men were arrested on Saturday as police continued to close in on the group.

Asked during an interview on BBC television whether some of the group were still at large, 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."

Prime Minister Theresa May said developments in the investigation into the bombing meant intelligence experts had decided to lower the threat level from its highest rating "critical", meaning an attack could be imminent, to "severe".

Police have issued a photograph of Abedi, a 22-year-old Briton born to Libyan parents, taken on Monday night before he blew himself up and said they believed he had assembled his bomb in an apartment in the city centre.

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

Abdedi was known to British security services before the bombing, the government has said, but Rudd declined to comment on exactly what had been known about him.

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 about 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," Rudd said.