The U.S. ambassador to South Africa accused the country of providing weapons to Russia.

JOHANNESBURG — The United States ambassador to South Africa has accused the country’s government of providing weapons and ammunition to Russia during its invasion of Ukraine, escalating the friction between the two countries over Russia and the war.

The ambassador, Reuben E. Brigety II, told reporters on Thursday that Washington has reason to believe that a Russian ship that docked near Cape Town at a South African naval base last December, the Lady R, “uploaded weapons and ammunition.”

“We are confident that weapons were loaded onto that vessel and I will bet my life on the accuracy of that assertion,” Ambassador Brigety said, according to a clip of his exchange with the news media that aired on the South African news channel Newzroom Afrika.

During an exchange with lawmakers in Parliament on Thursday, President Cyril Ramaphosa of South Africa said only that the matter regarding the Lady R “is being looked into.”

Later on Thursday, Mr. Ramaphosa’s spokesman, Vincent Magwenya, criticized the ambassador’s statement, saying that the two nations had agreed that an investigation into what happened with the Lady R would be allowed to run its course and that U.S. intelligence would provide any evidence it had.

“It is therefore disappointing that the U.S. Ambassador has adopted a counterproductive public posture that undermines the understanding reached on the matter,” Mr. Magwenya said in a statement.

Mr. Magwenya added that “no evidence has been provided to date to support” allegations about the purpose of the ship docking. But that assertion was contradicted by Senator Chris Van Hollen, a Democrat from Maryland.

Senator Van Hollen met with a South African government delegation that came to Washington last week, and he said in a statement that the visiting officials “indicated that they understood the gravity of our concerns regarding their increased military ties with Russia, and were taking seriously the evidence we have presented regarding transfers of weapons and ammunition to Russia.”

The South African officials indicated that they would hold accountable anyone complicit in the transfers, he added.

While the United States has called on its allies to condemn and isolate Russia, South African officials have refused to do so, citing the support that the African National Congress, the governing party, received from the Soviet Union in the long fight against apartheid and saying that they were maintaining a neutral stance on the fighting in Ukraine.

American and other Western allies, however, have said that South Africa has not been neutral and, in fact, has backed Russia. Ambassador Brigety’s allegation that Pretoria is providing ammunition to Moscow is the most pointed diplomatic jab yet in an increasingly tense relationship. He made the comments during a news conference in Pretoria that was open only to South African media outlets.

Soon after the ambassador’s remarks South Africa’s currency, the rand, which recently had been falling in value against the U.S. dollar, continued its plunge.

Ambassador Brigety’s comments came about a week after he visited Washington with a delegation of South African officials, who held high-level talks with the Biden administration and lawmakers. The delegation, sent by Mr. Ramaphosa of South Africa, sought to smooth over some of the differences between the countries.

John Steenhuisen, the leader of the Democratic Alliance, South Africa’s top opposition party, called the ambassador’s statement “a chilling and deeply troubling confirmation that President Cyril Ramaphosa and his government are actively involved in the Russian Federation’s war on Ukraine.”

Senator Jim Risch, the ranking Republican on the Senate Foreign Relations Committee, called on the Biden administration “to re-evaluate the scope and scale of our current engagement with South Africa’s government.”

The American government has hinted at retaliating against South Africa if it is found to have aided Russia’s war effort or helped it to evade sanctions. The United States has several options, including issuing sanctions and revoking trade privileges.

Lynsey Chutel contributed reporting.