SALMAN Abedi saw Muslim children being killed by Western military strikes in the Middle East and “wanted revenge,” his sister said, suggesting this was the motive for the suicide bombing in Manchester on Monday night, that killed 22 people.

Jomana Abedi told the Wall Street Journal: “He saw children - Muslim children - dying everywhere and wanted revenge. He saw the explosives America drops on children in Syria and he wanted revenge. Whether he got that is between him and God."

Last month, a US military strike was used to destroy what Washington said was a chemical weapons store. Six Syrian soliders were killed in the attack as well as nine civilians, including four children.

A Libyan anti-terror official also revealed how the 22-year-old terrorist had phoned his mother hours before the Manchester attack and told her: “Forgive me.”

Details of the intelligence agencies' knowledge of Abedi came as police hunting the "network" behind his attack said they had made "significant" arrests and seized "very important" items in raids linked to the investigation.

After chairing a meeting of the Cobra emergency committee, Theresa May said the terror threat level would remain at critical,meaning another attack is expected imminently.

In an indication of the level of counter-terrorism activity, a senior Whitehall source revealed that 18 plots had been foiled since 2013 in Britain, including five in just nine weeks since the Westminster attack in March this year.

It is understood the scale of the threat being dealt with by counter-terror agencies is "unprecedented" and intelligence officers faced "difficult professional judgements" about where to focus their investigations.

The source said: "MI5 is managing around 500 active investigations, involving some 3,000 subjects of interest[SOIs] at any one time.

"Abedi was one of a larger pool of former SOIs whose risk remained subject to review by MI5 and its partners.

"Where former SOIs show sufficient risk of re-engaging in terrorism, MI5 can consider reopening the investigation but this process inevitably relies on difficult professional judgements based on partial information."

It has emerged that Abedi, the British-born son of Libyan parents, had been banned from a mosque in Manchester after criticising an imam for "talking b******s" during a sermon critical of the so-called Islamic State terror group.

There have also been claims the authorities had been warned about concerns he was developing radical views.

The bomber’s father Ramadan and brother Hashim have been detained in Libya and another brother, Ismail, was arrested in Manchester on Tuesday.

While Abedi was born and raised in Manchester, his parents had arrived in the UK having fled the Gaddafi regime in Libya the early 1990s.

Protesting his son's innocence regarding the Manchester Arena bombing, Ramadan Abedi added: "I'm sure that Salman didn't carry out such an act."

While Mr Abedi said his son had seemed "normal" when he spoke to him five days before the atrocity, Jamal Zubia, a member of the large Libyan community in Manchester, claimed the parents were so concerned about their son's apparent descent into extremism that they took his passport.

He said: "The father had all the passports with him and was holding them."

But, Mr Zubia, explained, that Abedi had convinced them to return it, claiming he wanted to make a pilgrimage to Mecca.

But, in fact, he came back to the UK, via Duesseldorf, from Turkey four days before the bombing. One suggestion is that he had been in Syria.

Akram Ramadan, 49, part of the close-knit Libyan community in south Manchester, said Abedi’s brother Hashim, who he described as "the little boy", had once lived in Germany.

Mr Ramadan, who lived in the flat above Ismail, recalled how Abedi was banned from Didsbury Mosque: "There was a sermon about anti-Daesh[IS] and he stood up and started calling the Imam: 'You are talking b******.' And he gave a good stare, a threatening stare into the Imam's eyes. He was banned."

Asked about Abedi's motives for the attack, Mr Ramadan replied: "Something flipped him. Brain-washed; seen something, heard something. It is unbelievable."