The US military has deployed soldiers to Gabon amid fears of violent protests in neighbouring Democratic Republic of the Congo (DRC) after its presidential election.

US President Donald Trump told US Congress on Friday that the first of about 80 troops had arrived in Gabon on Wednesday to protect US citizens and diplomatic facilities should violence break out in DRC's capital Kinshasa.

Voters in the DRC went to the polls on December 30, two years after the election was first scheduled to be held, to elect the successor to President Joseph Kabila, who has been in power for 18 years.

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"The first of these personnel arrived in Gabon on January 2, 2019, with appropriate combat equipment and supported by military aircraft," Trump's letter to Congress read.

"Additional forces may deploy to Gabon, the Democratic Republic of the Congo, or the Republic of the Congo, if necessary for these purposes.

"These deployed personnel will remain in the region until the security situation in the Democratic Republic of the Congo becomes such that their presence is no longer needed."

The DRC's electoral commission is scheduled to release the provisional results of the presidential election on Sunday, but it has said there could be delays because of the slow arrival of tally sheets.

'Potential unrest'

Observers and the opposition say the election was marred by serious irregularities, the DRC's government says the election was fair and went smoothly.

Kabila's ruling coalition is backing his hand-picked successor Emmanuel Ramazani Shadary.

Observers and the international community have raised concerns that a disputed result could cause unrest, as was the case after the 2006 and 2011 elections.

Al Kitenge, CEO of London-based Innovation Task Force, said there was "huge risk of trouble in the coming days".

"We've had violence in the countryside for the past 40 years... Today everyone is worried because the violence might spread to Kinshasa. The people most at risk in such a case are civilians and we hope everything can be done to avoid that," he told Al Jazeera.

On Thursday, the US Department of State called on the electoral commission to ensure votes were accurately counted and threatened to impose sanctions against those who undermined the process or threatened peace and stability in the country.

Human Rights Watch (HRW) also warned against any manipulation of the results.

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"The African Union and other governments should make clear to Congo's leadership that any manipulation of the election results will have serious consequences," said Ida Sawyer, deputy Africa director at HRW.

"Rigged or fake vote tallies would only inflame an already tense situation and could have disastrous repercussions."

Large-scale ethnic violence broke out in Yumbi, in western DRC's Mai-Ndombe province, leaving at least 150 dead in a previously peaceful region, according to HRW.

Yumbi was among the three areas whose elections were postponed until March, in addition to Butembo and Beni, because of concerns over an Ebola outbreak and ethnic violence.