Russian President Vladimir Putin must publicly call for pro-Moscow rebels to lay down their arms and cut support to them, Kerry said, adding: "There are many concrete things that would make a difference on the ground."
Washington was "delighted" with Putin's decision, approved by parliament today, to revoke the blanket authorisation to intervene if ethnic Russians in eastern Ukraine were threatened.
"It is an important, a great step - but it can be reversed in 10 minutes," Kerry said.
Putin's failure to come up with real commitments on the ground in the "next days and weeks," runs the risk of tougher sanctions, Kerry warned after a NATO foreign ministers meeting, stressing that preparations for such measures were ongoing.
Many ministers made a similar point, arguing that Russia must unambiguously sign up to the Ukraine government's peace plan or face the consequences.
"We are counting on the positive signals that the Russian president is now sending being heard across the world and, above all, in Ukraine," Deputy Foreign Minister Grigory Karasin told Russian news agencies.
