The United States on Tuesday summoned Zimbabwe's ambassador after the ruling party insulted the US envoy in Harare and accused him of fomenting anti-corruption protests.
Patrick Chinamasa, spokesman for the ruling Zanu-PF party, on Monday called US Ambassador Brian Nicholls "a thug" and threatened to expel him.
"Comments from Zanu-PF -- while sadly not surprising -- are deeply offensive," said Tibor Nagy, the top US diplomat for Africa.
"We have summoned the ambassador of Zimbabwe to explain," he wrote on Twitter.
Zimbabwean President Emmerson Mnangagwa's administration has been cracking down on activists after a call for demonstrations on July 31 in the impoverished nation.
Among those arrested have been documentary filmmaker Hopewell Chin'ono, who helped expose a multimillion-dollar corruption scandal involving the procurement of coronavirus supplies.
The United States has criticized the arrests, with Nagy earlier calling on Zimbabwe to "uphold the people's constitutional rights to freedom of political expression and freedom of the press."
Zimbabwe's leaders have long alleged foreign plots as they wrestle with allegations of corruption and an economy that has struggled with eye-popping hyperinflation.
Brief hopes in the United States for an improvement in conditions in Zimbabwe after the 2017 ouster of veteran authoritarian leader Robert Mugabe have faded.
In a rare meeting last year, Nagy urged Mnangagwa to hold security forces accountable for violence against protesters.