The US has accused South Africa of secretly supplying arms to Russia.

The explosive accusation could spark a crisis for President Cyril Ramaphosa's government, which claims to have a neutral stance on Russia's ongoing invasion of Ukraine.

On Thursday, the US ambassador to South Africa, Reuben Brigety, told the media that the US government believes that weapons were loaded onto a Russian naval ship in December.

The Lady R was docked at Simon's Town naval dockyard near Cape Town.

"Among the things we noted was the docking of the cargo ship... which we are confident uploaded weapons and ammunition onto that vessel in Simon's Town as it made its way back to Russia," he said, as reported by News24.

"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," he said.

Ramaphosa responded to the accusation hours later, telling the National Assembly the matter is being "looked into".

The accusation, if proven to be true, could be very damaging for South Africa's relationship with the western world.

ActionSA leader Herman Mashaba wrote: "Actively supporting Russia's illegal war in Ukraine will harm South Africa's reputation with our international partners."

In the wake of the accusation, the rand crashed to its previous record low.

Ever since Russian President Vladimir Putin launched the invasion in February last year, Ramaphosa's government has been careful not to take sides.

However, the ANC's historical friendship with Russia and Putin has raised questions over how neutral it really is.

Last month, the government said it would not be arresting Putin upon his visit to SA in August, despite the International Criminal Court issuing a warrant for his arrest for war crimes.

A planned naval exercise held jointly with Russia and China in February was condemned by the DA and the foundation of the late Archbishop Desmond Tutu.

The foundation called the drills "disgraceful" and "tantamount to a declaration that South Africa is joining the war against Ukraine".