Somalia's newly-elected president on Tuesday thanked his US counterpart Joe Biden for ordering the redeployment of American troops to the Horn of Africa nation to combat the Al-Shabaab militant group.

The move reverses an order from Biden's predecessor Donald Trump, who in late 2020 pulled nearly all US forces from Somalia as he sought to wind down US military engagements abroad during his final weeks in office.

Hassan Sheikh Mohamud, who won a long-overdue election on Sunday to become Somalia's president, thanked Biden for sending US troops back to conduct operations against the Al-Qaeda-linked jihadists in his country.

"The #US has always been a reliable partner in our quest to stability and fight against terrorism," the president's office posted on Twitter on Tuesday.

The Somalia-based Al-Shabaab has been trying to overthrow the central government in Mogadishu for more than a decade, and has staged deadly attacks in neighbouring nations Kenya and Uganda.

The re-established US troop presence would involve fewer than 500 troops engaged in a "small persistent" effort in Somalia, a senior American official told reporters on Monday.

The official said the decision taken by Trump to withdraw US troops from Somalia went against the advice of senior military leaders and since then Al-Shabaab had only grown stronger.

The troops withdrawn from Somalia were rebased in Kenya and Djibouti. The US military command for Africa continued to conduct drone strikes in Somalia against Al-Shabaab targets.

By reinserting US troops, Washington will reduce the risks involved in back-and-forth mobilizations of forces that have been conducting counterterrorism operations inside Somalia.

The move would boost efficiency and the effectiveness of special operators, and allow for uninterrupted training periods with local partners.

Al-Shabaab were driven out of Mogadishu in 2011 by an African Union force, but still control swathes of countryside and the capacity to launch deadly attacks on civilian and military targets.

Somalia's international partners had repeatedly warned that delaying the presidential election distracted from the fight against Al-Shabaab, which in one high-profile attack killed 10 Burundian peacekeepers earlier this month.

Mohamud was elected Sunday in an indirect vote that ran over a year late, the process mired in political infighting that turned violent at times and sewed division in the fragile central government.