Alexanda Kotey, 37, was part of the Islamic State group that kidnapped and killed a number of Western and Japanese hostages in Iraq and Syria, according to a BBC report.

He pleaded guilty on all eight charges and now faces life in prison, the report informed.

"Kotey has been afforded due process and in the face of overwhelming evidence, he made the independent decision to plead guilty to his crimes," lawyer Raj Parekh, a member of the prosecuting team, told reporters outside the court, according to the report.

He had been held in Iraq by the US military before being flown to the United States to face trial on terrorism charges.

The four-member Islamic State cell was dubbed "The Beatles" because of their British accents, according to Reuters.

El Shafee Elsheikh, 32, a Sudanese-born British resident extradited to the United States with Kotey, also faces charges that include hostage-taking resulting in death and conspiracy to provide material support to terrorists, stated the Reuters report.

Kotey and Elsheikh were citizens of the United Kingdom, but the British government has withdrawn their citizenship.

They are accused of detaining or killing multiple foreign hostages, including US journalists James Foley and Steven Sotloff and aid workers Kayla Mueller and Peter Kassig.

They are also suspected in the killing of other hostages, including Alan Henning - a British taxi driver who was delivering aid - and Scottish aid worker David Haines, as well as two Japanese nationals.

The 'Beatles' also took part in graphic Islamic State videos posted online showing beheadings of foreign hostages.

Their torture methods included electric shocks with a taser, forcing hostages to fight each other and 20-minute beatings with sticks and waterboarding, according to a 24-page indictment, reported Reuters.

They had initially pleaded not guilty in October.

The US wanted to put them on trial but was lacking evidence.

BBC reported that both men were in a legal limbo for several months, held by the US military in Iraq, where the US Attorney General threatened to hand them over to local courts, known for their summary justice and executions.

But the US said it would not carry out the death penalty if the men were convicted in a US court, the report said.

Elsheikh is scheduled to go on trial in January, it added.

Kotey's plea deal, meanwhile, carries a minimum sentence of life without parole. However,  after serving 15 years in imprisonment,  he would be eligible to be transferred to the UK where he could be tried for any possible charges.

His sentencing is due to take place in March next year, the report informed.