The U.S. government has summoned Zimbabwean ambassador to that country, Innocent Mutembwa following Zanu PF acting spokesperson, Patrick Chinamasa's insults and threats to expel US ambassador to Zimbabwe, Brian Nichols.
The new developments could further strain tense relations between the former allies.
Addressing a press conference in Harare Monday, Chinamasa branded ambassador Nichols a thug, accusing him of meddling in Zimbabwe's internal affairs.
Zanu PF also accused Nichols of sponsoring planned July 31 protests by the opposition and local civic groups.
Chinamasa said Nichols and a "coterie of gangsters" should stop "mobilising and funding disturbances, coordinating violence and training insurgency. Our leadership will not hesitate to give him marching orders."
"Diplomats should not behave like thugs, and Brian Nichols is a thug," he said.
But in a message posted on Twitter Tuesday, US assistant secretary of state Tibor Nagy said they had summoned Mutembwa to explain Chinamasa's 'offensive' utterances.
"Comments from Zanu PF - while sadly not surprising - are offensive. We have summoned ambassador of Zimbabwe to explain," Nagy wrote.
The row between the US and its troubled former ally follows criticism from the superpower and other western powers of last week's arrests on opposition politician and organiser of the #31July protests Jacob Ngarivhume and freelance journalist Hopewell Chin'ono.
The two outspoken government critics have been accused of inciting the nation to revolt against President Emmerson Mnangagwa's rule.
They are currently languishing in remand prison awaiting trial on August 7.