South African President Cyril Ramaphosa has said an investigation is underway after the United States ambassador in Pretoria accused the country of having provided arms to Moscow despite its stated neutrality on Russia’s war in Ukraine.

The government is opening an independent inquiry led by a retired judge into the allegation, Ramaphosa’s office said in a statement on Thursday.

Earlier, the US ambassador to South Africa, Reuben Brigety, told local journalists in a briefing that Washington was confident a Russian vessel had uploaded weapons and ammunition from South Africa in December.

“Amongst the things we noted were the docking of the Russian cargo ship Lady R in Simon’s Town between December 6 and December 8, 2022, which we are confident uploaded weapons, ammunitions … as it made its way back to Russia,” Brigety said in a briefing to local journalists, as cited by the Reuters news agency.

“Arming of Russia by South Africa with the vessel … is fundamentally unacceptable,” Brigety said, adding that senior US officials had “profound concerns” about South Africa not respecting its professed policy of non-alignment and neutrality with regard to Russia’s war.

This “does not suggest to us the actions of a non-aligned country”, he said.

Washington has repeatedly warned countries against providing material support to Russia, cautioning that those who do may be denied access to the world’s most important markets.

Brigety made the remarks to journalists following his return to Pretoria after accompanying a high-level South African delegation to the US to address concerns about South Africa’s relationship with Russia.

South Africa conducted joint naval exercises with Russia and China in February – calling it routine – which the US and other Western powers expressed concern about at the time.

Brigety said on Thursday that the naval drills and other issues, in addition to the arms shipment, had contributed to concerns about South Africa’s allegiance.

South Africa is one of Russia’s most important allies on a continent divided over Moscow’s invasion of Ukraine, but it has maintained it is impartial on the conflict and abstained from voting on United Nations resolutions on the war.