But asked on Thursday who the men in unmarked uniforms were, Putin said they were Russian servicemen who "stood behind the back of Crimea's self-defense forces."
"They acted politely, but resolutely and professionally," he said. "There was no other way to hold the referendum in an open, honest and honorable way and allow the people to express their opinion."
He said part of the motives behind the annexation of Crimea was the need to counter what he said was NATO's intention to make Ukraine a member and sharply limit Russia's presence in the Black Sea region.
Putin insisted that protests in the east of Ukraine only involve locals. He denounced the Ukrainian authorities' decision to use the military to uproot the protests in the east as a "grave crime," adding that he told his Western counterparts urging him to help disarm protesters in the east that the Ukrainian government should first pull the army back.
"They are sending tanks, armored personnel carriers and cannons there!" he said. "Have they gone nuts?"
He expressed hope for the success of Thursday's talks in Geneva that bring together the United States, the European Union, Russia and Ukraine for the first time since the Ukrainian crisis erupted.
"I think the start of today's talks is very important, as it's very important now to think together about how to overcome this situation and offer a real dialogue to the people," Putin said.
Russia has demanded that the new government in Kiev, which replaced pro-Russian President Viktor Yanukovych, who fled to Russia following months of protests over his decision to spike a pact with the EU in favor of closer ties with Russia, move to transform the country into a loose federation. Ukraine has rejected the demand, but promised to give the regions more powers.