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 (South Africa 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 critics cite a number of recent incidents as evidence of a tilt towards the Kremlin.

- Diplomatic dilemma -

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".

Ambassador 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 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.

Asked in parliament about the accusation, President Cyril Ramaphosa on Thursday said the matter concerning the Lady R was "being looked into" and "in time we will be able to speak about it."

South Africa has been walking a diplomatic tightrope over the Ukraine conflict.

The country has strong economic and trade relations with the US and Europe.

But it also has ties with Russia dating back decades, to when the Kremlin supported the now-ruling African National Congress (ANC) in its fight against apartheid.

It is a member of BRICS -- a grouping bringing together Brazil, Russia, India, China and South Africa -- and has advocated for multilateralism as counterbalance to a US-led international order.

In March it was faced with a diplomatic dilemma after the International Criminal Court (ICC) issued an arrest warrant against Russian President Vladimir Putin, who is due to attend a BRICS summit in South Africa in August.

The warrant meant that Pretoria would have to detain Putin on arrival.

In response, last month Ramaphosa said the ANC had resolved that South Africa should quit the ICC -- before backtracking hours later citing what his office called a communications "error."

The rand, which has been softening against the dollar in recent days, dropped sharply Thursday, reaching reach its lowest point in three years, after news of the ambassador's remarks spread.