Ambassador Reuben Brigety told a media briefing the US was "confident" weapons and ammunition had been loaded onto a Russian freighter that docked at a Cape Town naval base in December.

"The arming of the Russians is extremely serious, and we do not consider this issue to be resolved, and we would like SA to [start] practising its non-alignment policy," Brigety was quoted as saying.

The US embassy did not immediately respond to a request for comment about the remarks, which were confirmed to AFP by a source at the meeting.

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 earlier this year, it held a controversial joint military exercise with Russia and China, which critics cited as evidence of a tilt towards the Kremlin.

The ambassador 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 6th to 8th December, 2022, which we are confident uploaded weapons and ammunition onto that vessel in Simon's Town as it made its way back to Russia," the envoy said.