JOHANNESBURG — The South African government said Thursday that it is investigating allegations, posted on a website, that the head of the state watchdog agency and opposition figures spied for the U.S.

The site, which is titled Africa Intelligence Leaks and contains no contact information, accused Public Protector Thuli Madonsela of being a CIA spy. Her office described the allegations as "ridiculous."

Julius Malema, leader of an opposition party that says it wants to redistribute resources to the poor, was also accused of working for American intelligence.

The U.S. Embassy in South Africa said it had no comment. The CIA also declined to comment.

The State Security Agency, South Africa's intelligence service, said it viewed the matter seriously.

"These allegations will impact negatively on our hard-won democracy and will undermine the independence and credibility of our democratic institutions," the agency said. It said it would work with other security departments to "determine the veracity of the allegations made."

Oupa Segalwe, spokesman for Madonsela's office, said the claims amounted to cybercrime and that authorities should find out who was responsible for them.

Madonsela released a report last year that said President Jacob Zuma had inappropriately benefited from state funding and should pay back some of the more than $20 million spent on alleged security upgrades at his private home. Madonsela received praise in some quarters, but her detractors accused her of posturing in the media and pandering to the political opposition.

Last year, Kebby Maphatsoe, the deputy defense minister, accused Madonsela of being a CIA spy, saying her office was being used to attack the ruling party, the African National Congress.

Madonsela's office demanded that the deputy minister produce evidence of his claims. The South African government distanced itself from Maphatsoe's comments and he apologized.

Malema is a former head of the ruling party's youth league who was kicked out and formed the Economic Freedom Fighters, which entered parliament in elections last year.

Mbuyiseni Ndlozi, spokesman for Malema's party, criticized the government decision to investigate the spying allegations.

"They should focus on matters of state security rather than looking for grounds to investigate opposition leaders," Ndlozi said.