The declaration, which was drafted without input from the United States, "can't be renegotiated" South African President Cyril Ramaphosa's spokesperson told reporters, reflecting strains between Pretoria and US President Donald Trump's administration, which boycotted the event.
"We had the entire year of working towards this adoption and the past week has been quite intense," spokesperson Vincent Magwenya said.
Ramaphosa, host of this weekend's gathering of Group of 20 leaders in Johannesburg, had earlier said there was "overwhelming consensus" for a summit declaration.
G20 envoys drew up a draft leaders' declaration on Friday and hours later, the White House said Ramaphosa was "refusing to facilitate a smooth transition of the G20 presidency" after initially saying he would pass the gavel to 'an empty chair.'"
"This, coupled with South Africa's push to issue a G20 Leaders Declaration, despite consistent and robust US objections, underscores the fact that they have weaponised their G20 presidency to undermine the G20's founding principles," said White House spokeswoman Anna Kelly.
Trump looks forward to "restoring legitimacy" to the group next year, when the US holds the rotating presidency.
At the last minute Argentina, whose far-right President Javier Milei is a close ally of Trump, quit the negotiations right before the envoys were about to adopt the draft text, South African officials said.
"Argentina, although it cannot endorse the declaration ... remains fully committed to the spirit of cooperation that has defined the G20 since its conception," its foreign minister Pablo Quirno said at the summit.
In explanation, Quirno said Argentina was concerned about how the document referred to geopolitical issues.
"Specifically it addresses the longstanding Middle East conflict in a manner that fails to capture its full complexity," he said.
The document mentions the conflict once, saying members agree to work for a just, comprehensive, and lasting peace in ... the Occupied Palestinian Territory."
Envoys from the G20 - which brings together the world's major economies - drew up a draft leaders' declaration on Friday without US involvement, four sources familiar with the matter said.
"It is a longstanding G20 tradition to issue only consensus deliverables, and it is shameful that the South African government is now trying to depart from this standard practice," a senior Trump administration official said on Friday.
The declaration used the kind of language long disliked by the US administration: stressing the seriousness of climate change and the need to better adapt to it, praising ambitious targets to boost renewable energy and noting the punishing levels of debt service suffered by poor countries.
The mention of climate change was a snub to Trump, who doubts the scientific consensus that global warming is caused by human activities. US officials had indicated they would oppose any reference to it in the declaration.
In opening remarks to the summit, Ramaphosa said: "We should not allow anything to diminish the value, the stature and the impact of the first African G20 presidency."
His bold tone was a striking contrast to his subdued decorum during his visit to the White House in May, in which he endured Trump repeating a false claim that there was a genocide of white farmers in South Africa, brushing aside Ramaphosa's efforts to correct his facts.
Trump said US officials would not attend the summit because of allegations, widely discredited, that the host country's Black majority government persecutes its white minority.
The US president had also rejected the host nation's agenda of promoting solidarity and helping developing nations adapt to weather disasters, transition to clean energy and cut their excessive debt costs.
The South African presidency on Saturday reiterated its rejection of a US offer to send the US charge d'affaires for the G20 handover.
"The president will not hand over to a junior embassy official the presidency of the G20. It's a breach of protocol that is not going to be accommodated," Magwenya said.
Lamola later said that South Africa would assign a diplomat of the same rank as a charge d'affaires to hand over the G20 presidency at the foreign affairs department.
