The 20-year-old brother of the Manchester bomber was allegedly part of a jihadist cell that was planning to target Martin Kobler (pictured above) - the United Nations special envoy to Libya, Britain's Sunday Telegraph newspaper reported.

Hashim Abedi, who was arrested this week in Tripoli following the suicide bomb attack at Manchester Area that killed 23 people, was understood by Libyan security services to be a "significant player" in the plot.

Citing diplomatic sources, the paper said the cell was in the late stages of building an explosive device that would be used on Kobler's convoy during a visit to Tripoli earlier this year.

'Plan interupted'

The Sunday Telegraph said the plot was interrupted before it could be carried out.

Kobler, who is German, was appointed as the UN's envoy to Libya in 2015 and has played a critical role in encouraging the country's many warring factions to work towards the creation of a unity government.

Read more: UN envoy - 'Inhumane things are happening in Libya'

Hashim Abedi is being detained by Libyan authorities along with his father in connection with the attack at the end of a concert by the pop singer Ariana Grande.

The pair are suspected of having links to the self-proclaimed "Islamic State" (IS) armed group, which claimed responsibility for Monday's bombing.

Hashim 'confessed'

According to Libyan authorities, Hashim has confessed that he and Salman were members of IS. British media reported that the brothers spoke by phone just 15 minutes before the bombing.

The paper said it was unclear under what conditions Hashim Abedi's confession was made.

Meanwhile, British officials have confirmed that Salman Adebi returned from Libya to Britain on May 18 via Germany. They said they were keen to receive any other information about his movements in the days before the attack.

Read more: Manchester bomber - A life on the move

Late on Saturday, police in Manchester released two CCTV images of Salman Abedi from the night he carried out the atrocity. Wearing a dark jacket, jeans, a baseball cap and glasses, the 22-year-old can be seen with a rucksack on his back, which is believed to have carried the explosives used in the attack.

Several arrests

Since Monday's bombing, a total of 11 people have been arrested in Britain on suspicion of terrorism-related offenses. Among them is a third brother, 23-year-old Ismail Abedi.

On Saturday, British authorities downgraded the country's terror alert level from "critical" to "severe" following several days of arrests.