The US ambassador to South Africa has accused the country of covertly providing arms to Russia – a charge that drew an angry rebuke from Pretoria.

Reuben Brigety told a media briefing on Thursday that the US believed weapons and ammunition had been loaded on to a Russian freighter that docked at a Cape Town naval base in December.

“We are confident that weapons were loaded on to that vessel and I would bet my life on the accuracy of that assertion,” Brigety said, according to a video of the remarks. “The arming of Russia by South Africa … is fundamentally unacceptable.”

President Cyril Ramaphosa’s office hit back, saying it was “disappointing” that Brigety had “adopted a counterproductive public posture” but announcing the launch of an inquiry into the affair.

A spokesman for the US state department later described the move as “a welcome step”. “The deeply concerning piece of this is the docking of a sanctioned Russian vessel at a South African naval port,” Vedant Patel told reporters in Washington.

He said the US would speak out against “any country taking steps to support Russia’s illegal and brutal war in Ukraine”.

South Africa has refused to condemn the invasion of Ukraine, which has largely isolated Moscow on the international stage. The country – an African powerhouse that also wields moral clout for its victory over apartheid – says it wants to stay neutral, and champions dialogue as the means to end the conflict.

But critics cite a number of recent incidents as evidence of a tilt towards the Kremlin. Earlier this year, it held a joint military exercise with Russia and China, and last month a sanctioned Russian military cargo plane landed at an air force base in the middle of the night to deliver what defence authorities described as “diplomatic mail”.

Brigety appeared to refer to a previously known episode, when the Lady R, a cargo vessel under western sanctions flying a Russian flag, docked at South Africa’s largest naval base.

“Among the things we noted was the docking of the cargo ship in the Simon’s Town naval base between 6 to 8 December 2022, which we are confident uploaded weapons and ammunition on to that vessel in Simon’s Town as it made its way back to Russia,” the envoy said, according to news outlet News24.

The US would like South Africa to start “practising its non-alignment policy,” he added.

The main opposition party, the Democratic Alliance, accused the government of trampling on South Africa’s values and interests “in favour of a global warmongerer and despot” and warned of “major consequences”.

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