Russian President Vladimir Putin says there is no need yet to send Russian troops into Ukraine.
But Russia reserves the right to use "all means" to protect citizens in eastern Ukraine, Putin said.
He denied Russian troops had besieged Ukrainians based in Crimea - they were pro-Russian "self-defence" forces.
Putin called the toppling of President Viktor Yanukovych in the capital Kiev an "anti-constitutional coup and armed seizure of power".
He said "militants" had plunged the country into "chaos". He also said Ukrainian "nationalists" and "anti-Semites" were roaming the streets of Kiev and other cities.
If Russian-speaking people in eastern Ukraine asked for Russia's help then Moscow would respond, he said.
"If we see this anarchy beginning in the eastern regions we reserve the right to use all means," he said.
In Crimea pro-Russian armed men and civilians are surrounding Ukrainian military bases - not Russian soldiers, he said.
"Local forces of self-defence" were responsible for taking over official buildings in Crimea, he said.
Ukraine says Russia has poured thousands of extra troops into Crimea in recent days.
Tensions remain especially high at Belbek airbase near Sevastopol, the port city which is the base of Russia's Black Sea Fleet.
Referring to the overthrow of Mr Yanukovych, Mr Putin said the ousted leader had agreed to all of the opposition's demands.
He insisted that Yanukovych was still the legitimate president, and accused the West of encouraging the street protests that had ousted him.
There were only three legal means to remove a president, he said: death, personal resignation or impeachment.
Yanukovych fled to Russia, and Putin told the news conference: "I don't think he has a political future."
Russia had helped him for "humanitarian" reasons, he said, "otherwise he'd just have been killed".