US Secretary of State calls on Putin to choose 'peaceful path' amid Ukraine crisis
"I strongly, strongly hope that we can keep this on a diplomatic and peaceful path, but ultimately, that's going to be President Putin's decision," Blinken said at the US embassy.
If Putin does not pursue diplomacy, Blinken warned, he will be choosing "confrontation and consequences for Russia".
Blinken will meet Russian Foreign Minister Sergei Lavrov on Friday in Geneva amid Western alarm over the deployment of tens of thousands of Russian troops to Ukraine's borders.
Blinken voiced concern that Russian movements may be far from over.
"We know that there are plans in place to increase that force even more on very short notice, and that gives President Putin the capacity, also on very short notice, to take further aggressive action against Ukraine," Blinken said.
Belarus, which also borders Ukraine and whose president Alexander Lukashenko is an ally of Putin, announced the arrival of Russian troops for what it described as drills.
Blinken said that Russia - which is demanding guarantees that Ukraine not be allowed into NATO - was acting with "no provocation, no reason".
Russia is looking "to challenge some very basic principles that undergird the entire international system and are necessary for trying to keep peace and security - principles like one nation can't simply change the borders of another by force," Blinken said.
"If we allow those principles to be violated with impunity, then we will open a very large Pandora's box. The entire world is watching what is happening here."