South Africa has provided weapons to Russia via a cargo ship that docked secretly at a naval base near Cape Town for three days in December, a senior official claimed on Thursday.
South African President Cyril Ramaphosa said an investigation was underway.
Ambassador Reuben Brigety said the U.S. was certain that weapons were loaded onto the vessel at the Simon's Town naval base and then transported to Russia, according to reports of his comments carried by multiple South African news outlets.
Ramaphosa was in Cape Town answering questions in Parliament when news of Brigety's comments broke.
When a lawmaker asked about the weapons, the president replied that "the matter is being looked into, and in time we will be able to speak about it."
Ramaphosa declined to comment further, citing the need for an investigation to play out.
The leader of the political opposition, John Steenhuisen, asked the president if South Africa was "actively arming Russian soldiers who are murdering and maiming innocent people?"
Steenhuisen also asked if Ramaphosa could confirm that "weapons of war" were loaded onto the Russian ship in Simon's Town, The lawmaker identified the vessel as the Lady R, a Russian-flagged cargo ship.
Brigety said earlier Thursday that South Africa's alleged arming of Russia during its invasion of Ukraine was "extremely serious" and called into question South Africa's supposed neutral stance in the conflict.
"Among the things we (the U.S.) noted was the docking of the cargo ship in the Simon’s Town Naval Base between the 6th and 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," Brigety was quoted as saying to reporters during a news conference in the South African capital, Pretoria.
Steenhuisen's party, the Democratic Alliance, raised questions earlier this year about a "mystery" Russian vessel making a stop at the Simon's Town base.
At the time, the South African government didn't comment publicly on the alleged incident, saying it needed to gather information. In late December, South African Defense Minister Thandi Modise said the ship appeared to be handling an "old order" for ammunition, and she indicated that arms were offloaded, not loaded onto the ship.
The South African government, a key partner for the U.S. in Africa, has stated numerous times it has a neutral position on the war in Ukraine and wants the conflict resolved peacefully.
But recent displays of its closeness to Russia opened Africa's most developed country to accusations that it has effectively taken Russia's side.
South Africa hosted Russian Foreign Minister Sergei Lavrov for talks in January, around a month after the alleged visit by the Lady R, giving him a platform to blame the West for the war in Ukraine.
Weeks later, South Africa allowed warships from the Russian and Chinese navies to perform drills off its east coast. The Russian navy brought its Admiral Gorshkov frigate, one of its navy's flagship vessels.
The South African navy also took part in the drills and characterized them as exercises that would "strengthen the already flourishing relations between South Africa, Russia and China."
South Africa's decision to stage those naval drills in February, which coincided with the one-year anniversary of the start of the war in Ukraine, raised "serious concerns" for the U.S., Brigety was quoted as saying on Thursday.
At the time of the drills, the South African armed forces said they were planned years ago before Russia's invasion of Ukraine.
South Africa also faces a diplomatic dilemma over a possible visit this year by Russian President Vladimir Putin, who is the subject of an International Criminal Court arrest warrant for alleged war crimes involving the abductions of children from Ukraine.
Putin is due to visit South Africa in August for a meeting of leaders of the BRICS economic bloc, which is made up of Brazil, Russia, India, China and South Africa.
South Africa is a signatory to the international war crimes court and therefore obliged to arrest Putin. But the government has indicated it will not detain the Russian leader and has threatened to leave the ICC instead.
South Africa's ruling African National Congress Party, which is led by Ramaphosa, sent a delegation to Moscow last month and spoke of strengthening ties with Russia, further straining the country's relationship with the U.S.