The US Emabssy in Harare says it will only maintain minimal staff on Wednesday
"As a result of the ongoing political uncertainty through the night, the Ambassador has instructed all employees to remain home (Wednesday)," the embassy in Harare said in a statement.
"The embassy will be minimally staffed and closed to the public... US citizens in Zimbabwe are encouraged to shelter in place until further notice."
The British foreign office earlier said it was "aware of reports of military vehicles moving on the outskirts of Harare" and said it was monitoring the situation closely.
Several armoured vehicles were seen driving down main roads outside the Zimbabwean capital on Tuesday, as tension erupted between Mugabe's regime and the military that has been a key buttress to his 37-year reign.
In an incendiary statement, Mugabe's ZANU-PF party accused army chief General Constantino Chiwenga of "treasonable conduct" for challenging Mugabe over the recent sacking of the vice president.
The public dispute has presented a major test of whether 93-year-old Mugabe, who has ruled since independence from Britain in 1980, still has a firm grip on power.