During a trip to Angola, Defense Secretary Lloyd Austin said Africa needs militaries that serve their citizens and not the other way round.

"That will remain a core principle of America's engagement with our African partners," Austin said in a speech in the capital, Luanda.

"So we will continue to invest in professional, civilian-led militaries... And we'll be candid with our partners when their security institutions fall short of those universal standards."

Austin did not name any country, but his comments appeared to be a clear reference to Niger, where military leaders overthrew a democratically elected president in July and the US has about 1,100 military personnel.

"When generals overturn the will of the people and put their own ambitions above the rule of law, security suffers," he said, decrying "autocrats" who "undermine free and fair elections".

Earlier this week, Austin said the US would "evaluate" its next steps on the Niger crisis after France announced a full troop withdrawal as demanded by the military leaders.

Austin was in Angola on the last leg of a three-country tour, including stops in Djibouti and Kenya, aimed at "strengthening partnerships" on the continent, where China and Russia have enjoyed rising clout.

Niger is among several nations to have undergone coups since 2020, along Burkina Faso, Guinea, Gabon and Mali.

The latter has since pivoted towards Moscow, bringing in mercenaries from the Wagner group.

"Africa deserves better than autocrats selling cheap guns, pushing mercenary forces like the Wagner Group, or depriving grain from hungry people all around the world," Austin said, in a reference to the Kremlin.

In Luanda, he met President Joao Lourenco and said Washington would deepen cooperation with the country "on military modernization, training, maritime security, and medical readiness."

The visit, the first by a US Secretary of Defense, underscored Angola's diplomatic shift towards the west under Lourenco.

One of the largest oil exporters in Sub-Saharan Africa, the country long held close ties to China and Russia.

Its ruling party was supported by the Soviet Union in the civil war against US-backed rebels.

"Over the past few years, America's relationship with Angola has taken huge strides forward," Austin said.