"The health of the president is entirely normal. I spoke to the president this morning," Vice President Guy Scott told parliament.
He also denied international reports that the president had been hospitalised.
"He has not received any emergency or specialist medical treatment," Scott said.
This is the second time in a week that the Zambian government has had to deny that the ailing president had died.
The first time it was Sata himself who made a rare public appearance on September 19 to tell parliament, "I am not dead."
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Scott gave parliament no explanation of why Sata failed to make his scheduled address to the general assembly on Wednesday night.
Sata's son Mulenga, who called a news conference to also deny reports of his father's death or hospitalisation, said the president would issue a statement in New York.
Zambian Foreign Minister Harry Kalaba was scheduled to speak to the assembly today.
Despite repeated denials that the president is ill, analysts say a power struggle is already underway behind the scenes for Zambia's top job.