But Manchester United fans stood together in defiant mood as their team's triumph in European football's Europa League final brought some much-needed smiles to a city still in pain.

The club dedicated their trophy to those killed, while manager Jose Mourinho said they would gladly exchange it if it could bring their lives back.

According to photographs from the scene of Monday's attack at the 21,000-capacity Manchester Arena, Abedi's device appeared to be fairly sophisticated.

Images obtained by The New York Times newspaper showed a detonator that bomber Salman Abedi was said to have carried in his left hand, shrapnel including nuts and screws, and the shredded remains of a blue backpack.

But after of the bomber's identity and details of the probe were leaked, the intelligence-sharing relationship between close allies London and Washington was left rocking.

"We are furious. This is completely unacceptable," a government ministry source said of the images "leaked from inside the US system".

The National Counter Terrorism Policing body said the breach of trust caused great "damage" and "undermines our investigations".

University dropout Abedi, 22, grew up in a Libyan family that reportedly fled to Manchester to escape the now-fallen regime of Libyan dictator Moamer Kadhafi.

His father Ramadan and younger brother Hashem were detained in Libya, authorities there said.

A spokesman for the Deterrence Force, which acts as Libya's Government of National Accord's police, said the brother was aware of Abedi's plan and the siblings were both members of the Islamic State jihadist group.

The attack was the latest in a series of deadly incidents across Europe claimed by IS that have coincided with an offensive on the group's redoubts in Syria and Iraq carried out by US, British and other Western forces.

Hashem had been "under surveillance for a month and a half" and "investigation teams supplied intelligence that he was planning a terrorist attack in the capital Tripoli", the Deterrence Force said on its Facebook page.

A relative told AFP that Abedi had travelled to Manchester from Libya four days before the bombing.

British officials said Abedi had been on the radar of the intelligence community before the massacre.

"It's very clear that this is a network that we are investigating," Manchester police chief Ian Hopkins told reporters.

Police announced two new arrests Thursday in their probe, bringing the total to eight people in custody in Britain. A woman detained on Wednesday was released without charges.

Early Thursday, police said they conducted a controlled explosion in the south of Manchester where they were carrying out searches in the Moss Side area connected to the attack.

Elders at the south Manchester mosque believed to have been frequented by Abedi insisted that his actions were wholly alien to their preaching, and pointed the finger at online radicalisation.

"This act of cowardice has no place in our religion," said Fawzi Haffar, a trustee at the Didsbury mosque.

French Interior Minister Gerard Collomb said Abedi had "likely" been to Syria after the trip to Libya, citing information provided by British intelligence services to their counterparts in Paris.

"In any case, the links with Daesh are proven," he said, using a term for IS.

Britain's terror threat assessment has been hiked to "critical", the highest level, meaning an attack is considered imminent.

Armed troops were sent to guard key sites, a rare sight in mainland Britain.

The attack was the deadliest in Britain since 2005 when four Islamist suicide bombers attacked London's transport system, killing 52 people.

May was to hold a meeting of Britain's emergency committee on Thursday before heading to Brussels.

A national minute's silence was to be observed at 11:00am (1000 GMT) for those killed and the dozens seriously wounded.

After the NATO summit, May is heading to the G7 gathering in Italy on Friday. However, she will curtail her trip, returning home late Friday, missing the Saturday talks in Sicily.

The bombing came just over two weeks before a snap general election set for June 8. Campaigning will resume on Friday.

A total of 64 people are being treated in hospital, including 20 in critical care, medical officials said. Twelve of the injured are aged under 16. One of those killed was an eight-year-old girl. (AFP)