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"Today Russia seeks to change the security landscape of Eastern and Central Europe," Kerry said in a speech in Washington referring to Russia's occupation of Crimea and the threat it posed to eastern Ukraine.
"Whatever path Russia chooses, the United States and our allies will stand together in our defence of Ukraine," Kerry said.
"And most important, together we have to make it absolutely clear to the Kremlin that NATO territory is inviolable. We will defend every single piece of it," he added.
In a call to NATO members, Kerry said unity was vital and members' defence budgets could not be allowed to shrink.
"Together we have to push back against those who want to change sovereign borders by force," Kerry said.
"We find ourselves in a defining moment for our trans-Atlantic alliance; Our strength will come from our unity," he said. "This moment is about more than just ourselves - the fact is that our entire model for global leadership is at stake."
Kerry said the United States and its European allies had to work together to ensure European countries were not dependent on Russian energy supplies.
"We can deliver greater energy independence if we help to diversify the energy sources available to European markets," he said in the address at a Washington think tank attended by ministers from several EU and NATO countries.
He called the crisis in Ukraine "a wake up call" to accelerate work "to promote a stronger, more prosperous trans-Atlantic community."