UK officials have said they only recently learned that Libyan authorities monitored Manchester suicide bomber Salman Abedi for more than a month before the attack he perpetrated, something the Libyans say he started planning in December.

In Britain, police and intelligence services have been piecing together 22-year-old Abedi's movements in the days leading up to the May 22 attack in which 22 people died. They are still searching a rubbish dump outside Manchester, looking for the blue suitcase he was seen with in a CCTV image in the hours before the attack.

"We've learned that he spent a month in Libya with his family, and was being tracked by Libyan intelligence," said a United Kingdom official who requested anonymity.

The BBC, meanwhile, quoted Ahmed Ben Salem, a spokesman for Libya's Special Deterrence Force, as saying the attack had been planned as far back as December. He claimed the force's agents had gathered important information about Abedi's contacts, both in Libya and the UK.

It wasn't clear whether the Libyans had attempted to share that information with UK intelligence services.

Libyan security officials told the BBC they had better contacts with the Central Intelligence Agency in the United States, and a general said Libya also wanted improved ties with the UK in order to avoid future atrocities, such as the Manchester bombing in which mainly young people died during a suicide attack at the end of an Ariana Grande pop concert.

Libyan officials were quoted as saying Abedi's brother, Hashem, bought the materials for the bomb while in the UK.

UK officials said Abedi assembled the bomb alone in his Manchester apartment.

Hashem and his father are being detained and questioned by the Libyan Special Deterrence Force.

Meanwhile, Mark Rowley, head of national counterterrorism policing in the UK, wrote in The Times newspaper that closer partnerships were needed, both with other countries and at home, to ensure the appropriate sharing of expertise and information.

Rowley wrote that five active plots had been foiled since March, and 18 since mid-2013. Police and officers from the UK's MI5 intelligence service have 500 active investigations involving some 3,000 individuals regarded as posing the biggest threat.

"There are another 20,000 who we continue to be concerned about," he wrote. "Some of these, we will decide to move into the priority group. Others may move from radical to would-be terrorist very quickly. These numbers have grown. There is more overseas influence from terrorist groups and an internet going darker (more impenetrable). That makes it harder to look at those we are concerned about."