and threatened additional sanctions against Russian interests.
Speaking in London alongside UK Foreign Secretary Philip Hammond, Kerry said Russia was "continuing to do land-grabbing in Ukraine" even as it professed to support peace efforts.
Russia's actions are "just simply unacceptable, so we are talking about additional sanctions, additional efforts," Kerry said.
"We are confident that over the next few days we are going to make it clear that we are not going to play this game, not going to sit here and be part of this extraordinarily craven behavior at the expense of the sovereignty and integrity of a nation.
"This is behavior that is completely counter to everything that the global community has worked to achieve and put in place ever since World War II, and I'm confident that the United Kingdom, the United States and others are prepared to stand up to it."
He also blasted Moscow's repeated denials that it is involved in arming the separatists and sending its own troops over the border.
In this age of technology and satellites, Kerry said, "there is no secret" over what is taking place. "We know to a certainty what Russia has been providing to the separatists, how Russia is involved with the separatists," he said.
Kiev and a number of Western leaders have steadfastly accused Russia of failing to rein in separatists and of continuing to arm, supply and train them. European nations, the United States and others have imposed financial sanctions on Russian interests in hopes of pressuring President Vladimir Putin to help end the hostilities.
Russia, which seized Ukraine's Crimean peninsula in March, denies any direct involvement in Ukraine and says any Russian soldiers in Ukraine are volunteers fighting during their vacation time.