Nelson Mandela remained in critical condition on Monday, after his condition deteriorated in hospital over the weekend.
"The doctors are doing everything possible to ensure his well-being and comfort," President Jacob Zuma told a news conference on Monday.
Zuma and African National Congress Deputy President Cyril Ramaphosa visited the anti-apartheid icon at the Pretoria hospital on Sunday and were briefed on his condition by the medical team. They asked the world to pray for the 94-year-old former president and his family "during this difficult time."
Mandela, who has been in the hospital with a lung infection since June 8, had been in "serious but stable" condition but was downgraded over the weekend, officials said.
In Washington, the White House National Security Council spokeswoman noted the latest reports from the South African government about the Nobel Peace Prize laureate's worsening condition.
"Our thoughts and prayers are with him, his family and the people of South Africa," spokeswoman Caitlin Hayden said.
In the statement, Zuma addressed reports that the ambulance that took Mandela to the hospital earlier this month broke down on the way, saying that the delay did not compromise his health.
"There were seven doctors in the convoy who were in full control of the situation throughout the period. He had expert medical care," his statement said.
"The fully equipped military ICU ambulance had a full complement of specialist medical staff including intensive care specialists and ICU nurses. The doctors also dismissed the media reports that Madiba suffered cardiac arrest. There is no truth at all in that report."