U.N. Secretary-General Ban Ki-moon, in a statement issued by his spokesman, said he was "troubled" by circumstances leading to this week's resignation of Malian Prime Minister Cheick Modibo Diarra.
Diarra resigned after he was arrested in his home in Bamako by soldiers who supported a coup this year.
British Foreign Office Minister for Africa Mark Simmonds expressed deep concern about political developments in Mali.
"The rapid appointment of a new government of national unity to take forward the political road map toward democratic elections and the reunification of Mali is critical to further progress," he said in a statement.
Military leaders had said Diarra was in the way of political developments in the country. Foreign and Islamic rebels this year claimed authority over northern Mali. U.S. officials had said they were worried the region may turn into an al-Qaida breeding ground given its lawless state.
African leaders have formulated a plan for military intervention, though U.N. officials had said force should be used as a last resort.