"It certainly would be a welcome step," State Department spokesman Vedant Patel told reporters of the promise of a probe made by a spokesman for President Cyril Ramaphosa.

"The deeply concerning piece of this is the docking of a sanctioned Russian vessel at a South African naval port," Patel said.

He said the United States would speak out against "any country taking steps to support Russia's illegal and brutal war in Ukraine."

The US ambassador to Pretoria, Reuben Brigety, earlier Thursday said that the United States believed weapons and ammunition had been laden onto a Russian freighter that docked at a Cape Town naval base in December.

The envoy called the alleged shipment by South Africa, which has stayed neutral over the Russian invasion of Ukraine, "fundamentally unacceptable."

Patel, asked in Washington about the ambassador's remarks, took a more conciliatory tone, saying that the United States would "engage on this directly with our South African partners."

"We continue to be committed to our affirmative agenda with our South African partners," Patel said, pointing to cooperation on public health, climate and trade.

He declined to say if there would be any repercussions for South Africa, after repeated US warnings to China not to provide arms to Russia.