"This is clearly an important moment in the investigation," said British Prime Minister Theresa May. "I hope it is a welcome step for the loved ones of all the victims."

Salman Abedi, who set off the bomb at the end of the concert on May 22, 2017, died in the explosion, which also wounded hundreds. British authorities have sought Hashem's extradition for nearly two years, saying he was involved in planning the lethal attack.

Ahmed Bin Salem, a spokesman for the Tripoli-based Special Deterrence Force, said they extradited Hashem from Libya following a court order. The militia is allied with a UN-recognised government in the Libyan capital.

Both Hashem, who was then 18, and the boys' father, Ramadan Abedi, were detained by Libyan forces shortly after the attack. Hashem told investigators that both he and his brother belonged to the Islamic State group and that he knew about the attack, the Special Deterrence Force said at the time. The father was later released without charges.

The arrest warrant for Hashem was issued by a Westminster Magistrates' Court judge and approved by Home Secretary Sajid Javid.

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Manchester police notified the victims' families earlier on Wednesday that the long-awaited extradition was in process. It was slowed by the complicated political and military situation inside Libya.

Libya slid into chaos following the 2011 uprising that toppled and killed dictator Muammar Gaddafi . The country has since become a haven for rival armed groups.

The Tripoli government is seeking Western support as it battles forces loyal to Khalifa Hifter, a veteran Libyan general who launched an offensive against the capital in April. The Special

Deterrence Force is one of several militias loosely aligned with the government that are battling Hifter's forces.

with AP