The ultimatum came as ministers at a European security conference urged a peaceful end to the stand-off.
Prime Minister Mykola Azarov defended his government's handling of the crisis, which erupted after Kiev declined a trade deal with the European Union.
Mr Azarov also clashed with Germany's foreign minister over charges police used excessive force against the protesters.
"Nazis, extremists and criminals cannot be, in any way, our partners in 'eurointegration'," Mr Azarov is quoted as telling Germany's Guido Westerwelle, referring to protesters who have blockaded or occupied government and public buildings.
Mr Westerwelle, who is in Kiev for a meeting of foreign ministers at the Organization for Security and Co-operation in Europe (OSCE), expressed concern about police behaviour. He said: "Recent events, in particular the violence against peaceful protesters in Kiev worry me greatly.
"The way Ukraine responds to the pro-European rallies is a yardstick for how seriously Ukraine takes the shared values of the OSCE," he added, echoing comments from other EU ministers.
The crisis has exposed a tug-of-war in Ukraine, which has oscillated between the EU and Moscow since the Orange Revolution overturned the post-Soviet political order nine years ago.