The Ukrainian crisis perched of the brink of war Sunday, as Ukraine mobilized its military and called up reservists in response to Russia’s incursion into the Crimea and fears of an all-out Russian invasion.

But United States Secretary of State John Kerry issued a stiff warning to Russia’s President Vladimir Putin over the Ukrainian crisis Sunday morning, calling the Russian action an “incredible act of aggression” that the U.S. and other Western powers will “go the hilt” to stop.

Accusing Putin of trying to use the Ukranian crisis “to annex Crimea,” Kerry warned the Russian leader, “Russia is going to lose, the Russian people are going to lose, and he’s going to lose all of the glow that came out of the Olympics, his $60 billion extravaganza.”

Kerry did not mention military options Sunday. Instead, he said that after conversations with other Western leaders the day before, the West will “isolate” Russia on the world stage for using force against Ukraine, banning visas for Russians, freezing bank accounts and taking other measures to strangle Russia’s already-wobbling economy.

“The ruble is already going down. Russia has major economic challenges,” Kerry said. “I can’t imagine that an occupation of another country is something that appeals to a people who are trying to reach out to the world, and particularly if it involves violence.”

In Russia, small anti-war rallies in Moscow were broken up by riot police with at least 361 protesters detained or arrested. At the same time, an estimated 27,000 Muscovites too to the street in rallies sanctioned by Russia’s government, in support of military action to quell the Ukrainian crisis.

Putin claims that the Ukrainian Russian population faces “real threats” after the recent protests which ousted pro-Russian Presiden Viktor Yunakovych, and that military action is aimed at protecting those people.

But one protester who got through the American media by phone before being cut off, disputed the official reason for Russia’s potential military action in the Ukrainian crisis.

“It’s too early to talk about protecting the interests of Russians in Ukraine, since we don’t see any real threat to them yet,” said college teacher Mikhail Daniel. “I think the the reasons for this intervention are about the ambitions of the Russian state.”

Russian troops have blocked access to three Ukraine military bases in Crimea, according to a spokesperson for the Ukraine Defense Ministry, Vladislav Seleznyov.

But there has so far been “no open confrontation between Russian and Ukrainian military forces in Crimea,” he said.

Ukraine interim Prime Minister Arseniy Yatsenyuk characterized the Ukrainian crisis in dire terms. “This is a red alert. This is not a threat. This is actually a declaration of war to my country,” he said. “We are on the brink of disaster.”