Foreign minister Naledi Pandor described the request from the liberal Democratic Alliance (DA) as "astounding" on Monday, after President Cyril Ramaphosa accused the leading opposition party of seeking to "mortgage" the nation to foreign powers.
"It's clear they have something to hide, given their ferocious response to what is a standard request worldwide," DA shadow foreign minister Emma Louise Powell told AFP.
The quarrel points to mounting political tensions ahead of the May 29 vote.
After three decades in power, the African National Congress (ANC) risks losing its parliamentary majority for the first time since the advent of democracy.
Last week, the DA wrote to US Secretary of State Antony Blinken, asking for international help "to ensure the integrity" of the vote and "safeguard against any attempts to disrupt the democratic process."
The message didn't go down well with the government, which has had an at times rocky relationship with Washington amid accusations it sided with Russia over the war in Ukraine.
"To write such a letter... is rather surprising and a demeaning attitude and statement about South Africa," Pandor told state broadcaster SABC.
Over the weekend, Ramaphosa labeled it "disingenuous," saying international observers have monitored the elections since white-minority rule ended in 1994.
"We have our regional organizations... and indeed we have the United Nations as well, they always come to monitor our elections. In fact we invite them," he told journalists.
Struck a nerve
Plagued by corruption and mismanagement allegations amid high rates of unemployment, poverty and crime, the ANC is expected to drop below 50 percent of the vote for the first time.
That would force it to find a coalition partner to remain in power.
But some polls are fueling opposition hopes to unseat it altogether.
A survey of over 1,500 registered voters by the Brenthurst Foundation and the SABI Strategy Group published Sunday put the ruling party at 39 percent with the DA trailing on 27 percent.
The DA, which has struggled to escape a white, middle-class image and win over black voters, has struck a coalition agreement with almost a dozen smaller parties. Together they are polling at 33 percent, according to the survey conducted in February.
"We have struck a nerve and we will not be deterred," Powell said of the letter.
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