Describing the events in Ukraine as a "wake-up call," the top American envoy cautioned that "We cannot continue to allow allied defense budgets to shrink," a refrain repeated oftentimes by U.S. officials over the years.
"Clearly, not all allies are going to meet the NATO benchmark of two percent of GDP overnight or even next year," he said in remarks delivered at the Atlantic Council think tank in Washington D.C., adding "But it's time for allies who are below that level to make credible commitments to increase their spending on defense over the next five years."
He once again alleged that Russia has taken "not one single step" to help to de-escalate tensions in eastern Ukraine but rather done to the contrary.
Pro-Russia militants are continuing to occupy government buildings in some cities in Ukraine's east and refusing to release seven military inspectors from the Organization for Security and Cooperation in Europe.
On Monday, the U.S. and the European Union slapped fresh sanctions on more Russian officials and entities, and threatened to target its key sectors of economy in the event of further destabilizing moves in eastern Ukraine.
Kerry alleged that Russia seeks to change the security landscape of East and Central Europe through its integration of Crimea and subsequent destabilization of eastern Ukraine.
"So we find ourselves in a defining moment for our transatlantic alliance, and nobody should mistake that," he said. "And we are prepared to do what we need to do and to go the distance to uphold that alliance."
"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.
Washington has sent some 600 troops to Poland and the Baltic states in preparation for a series of military drills.
In his remarks, Kerry also urged European nations to reduce their dependence on Russian gas with "a sense of urgency," and move fast to complete the Transatlantic Trade and Investment Partnership negotiations aiming to create the world's largest free- trade area.
Russia has rejected suggestions that it is orchestrating events in eastern Ukraine, insisting it has no plan to invade that vast region.