In a briefing to local journalists in Johannesburg on Thursday, Brigety said that the US was confident that a Russian cargo ship, Lady R, picked up weapons and ammunition in South Africa in December 2022.
The US has been warning countries against providing material support to Russia and has threatened to impose sanctions on them.
The statement by Brigety has caused concern among currency traders and the South African government.
The rand and South Africa’s 2030 government bond have extended losses as traders are worried about possible Western sanctions against South Africa.
South African President Cyril Ramaphosa did not confirm or deny the shipment but said that the government was looking into the matter.
“The matter is being looked into and in time we will be able to speak about it,” said Ramaphosa during a parliamentary session when an opposition leader asked him about the issue.
Brigety expressed his profound concerns about South Africa’s alleged assistance to Russia, despite Pretoria’s professed policy of non-alignment and neutrality.
“Arming of Russia by South Africa with the vessel… is fundamentally unacceptable,” Brigety said.
He added that the docking of the Lady R in Simon’s Town was a clear indication that South Africa had violated its neutrality policy.
“This does not suggest to us the actions of a non-aligned country,” said Brigety in a recording of the briefing heard by Reuters.
South Africa conducted joint naval exercises with Russia and China in February, which the US and other Western powers expressed concern about at the time.
Brigety said that the naval drills and other issues had contributed to concerns about South Africa’s allegiance.
South Africa is one of Russia’s most important allies on a continent divided over its invasion of Ukraine.
However, South Africa has maintained that it is impartial on the conflict and has abstained from voting on UN resolutions on the war.
The US State Department did not immediately respond to a request for comment.
The US embassy in South Africa and the US Treasury Department declined to comment on the matter.
A spokesperson for South Africa’s presidency said that it would issue a statement later.
The matter is expected to have far-reaching implications for South Africa’s relationship with the West and Russia.