Two Pakistani brothers held by the United States at Guantanamo Bay military prison for two decades were freed by U.S. officials and returned home on Friday, officials said.

US authorities have released two Pakistani brothers, Abdul and Mohammed Rabbani, from the military prison at Guantanamo Bay after over 20 years of detention. 

They will be reunited with their families after formal questioning by Pakistani authorities, according to security officials and a Pakistani senator.

Pakistan arrested Abdul and Mohammed Ahmad Ghulam Rabbani on suspicion of their links to Al-Qaeda in 2002 in Karachi, the country's largest southern port city.

Abdul was accused of working for avowed 9/11 mastermind Khalid Sheikh Mohammed (KSM) and operating an Al-Qaeda safe house in Karachi, but his detainee assessment indicates he was not believed to have had 'specific insight into Al-Qaeda operational plans'. 

Mohammed was accused of recruiting his older brother into extremist circles. He is believed to have organized travel and funds for KSM and Abd al-Rahim al-Nashiri, the mastermind of the October 2000 suicide bombing of the USS Cole missile destroyer, which left 17 US sailors dead.

They were never charged with any crimes during 20 years in U.S. custody.

After about 18 months in an Afghan detention site, they were moved to Guantanamo Bay in 2004, where they say they were subject to torture.

U.S. military records note the brother provided 'little intelligence of value' and did not recant statements during interrogations on grounds they were obtained by physical abuse.

The two brothers were approved for transfer in 2021 and it is unclear why they remained in prison for so long.

At last, they arrived at an airport in the capital, Islamabad on Friday.

The US Defence Department announced their repatriation in a statement the day before.

Pakistani senator Mushtaq Ahmed Khan, the chairman of the human rights committee in the upper house of Pakistan's Parliament, tweeted on Friday that the two brothers had reached Islamabad airport.

He said the men were 'innocently imprisoned in Guantanamo Bay for 21 years. There was no trial, no court proceedings, no charges against them. Congratulations on their release. Thank you Senate of Pakistan', he wrote on Twitter.

Mr Khan told the Associated Press he expected the men would soon be reunited with their families.