UN Secretary-General Antonio Guterres has called on all sides in Zimbabwe to show "restraint" after the country's military took control and President Robert Mugabe said he was under house arrest.
Guterres is monitoring the situation and "appeals for calm, non-violence and restraint," said UN spokesman Farhan Haq.
Guterres "stresses the importance of resolving political differences through peaceful means and dialogue, and in line with the country's constitution," he added.
The UN chief noted that the Southern African Development Community (SADC) was making efforts to end the crisis.
South Africa's government said President Jacob Zuma had spoken by telephone to Mugabe, who "indicated that he was confined to his home but said that he was fine."
Mugabe, 93, has ruled Zimbabwe since its independence from Britain in 1980.
Meanwhile, the head of the African Union on Wednesday said the crisis in Zimbabwe "seems like a coup" and called on the military to halt their actions and restore constitutional order.
Alpha Conde, who is also Guinea's president, said the AU condemned the actions of top brass in the southern African nation as "clearly soldiers trying to take power by force".
"The African Union expresses its serious concern regarding the situation unfolding in Zimbabwe," a statement sent to AFP said, expressing support for the country's "legal institutions".
The African body further demanded "constitutional order to be restored immediately and calls on all stakeholders to show responsibility and restraint," it added.
The Zimbabwean military took control of the country on Wednesday after a struggle to succeed 93-year-old President Robert Mugabe erupted in public, culminating in the sacking of the vice president, Emmerson Mnangagwa.
Mugabe is under house arrest, according to a conversation reported by President Jacob Zuma of South Africa.
Mugabe "indicated that he was confined to his home but said that he was fine," the South African government said in a statement.