Three Americans convicted for their role in a failed coup in the Democratic Republic of Congo last year have been sent home to the US to serve the rest of their jail terms.

The three were originally sentenced to death by a military court before their sentences were commuted to life in prison last week.

US Department of State spokesperson Tammy Bruce said the three were "in our custody".

The repatriation comes as the US and DR Congo explore a deal to exploit the central African country's huge mineral wealth.

Last week, US President Donald Trump's senior adviser for Africa visited DR Congo and confirmed that the two countries were in talks about minerals and said it could involve "multibillion-dollar investments".

DR Congo has large deposits of coltan and cobalt, used in electronic equipment and batteries for electric cars, which are currently largely extracted by Chinese mining companies.

The three American convicts - Marcel Malanga Malu, Tylor Thomson and Zalman Polun Benjamin - left DR Congo on Tuesday to serve the remainder of their sentences in the US, said Congolese presidential spokesperson Tina Salama.

They were escorted to N'Djili International Airport in Kinshasa in "strict compliance with legal procedures", said DR Congo's presidency.

The transfer "is part of a dynamic of strengthening judicial diplomacy and international cooperation in matters of justice and human rights" between DR Congo and the US, the presidency added.

The Americans were among 37 people sentenced to death last September by a military court.

Jean-Jacques Wondo, a dual Congolese and Belgian citizen who was also sentenced to death, was in February transferred to Belgium because of ill-health.

It is not clear if the other convicts, who include a Briton, a Belgian and a Canadian national, will also have their sentences commuted.

They were accused of leading an attack on both the presidential palace and the home of an ally of President Félix Tshisekedi last May. Later they were convicted of criminal conspiracy, terrorism and other charges, which they denied.

The suspected leader of the plot, Christian Malanga, a US national of Congolese origin, was killed during the attack, along with five others. Hi son, Marcel Malanga Malu, is among those sent home on Tuesday.

Details of the prisoner transfer agreement were not immediately clear but the State Department said it was aware of the repatriation done in collaboration with the US embassy in Kinshasa, the DR Congo capital.

But legal experts say it's unlikely the US will release them or shorten their sentences, reports the AP news agency.

The Department of State spokesperson said that the US condemned the armed attacks and supported DR Congo's bid to hold the convicts accountable, but she also sought "consistent, compassionate, humane treatment and a fair legal process".

Joseph Szlavik-Soto, a lobbyist working for the Congolese government, told Reuters news agency that it had agreed to pay for the damage caused by protesters who attacked the US embassy and other missions earlier this year. The US officials are yet to comment on the matter.