After meeting Ukrainian President Volodymyr Zelenskiy, Blinken will then travel to Berlin for talks with allies before going to Geneva to meet Russian Foreign Minister Sergei Lavrov after negotiations last week produced no breakthrough.
Russia has massed tens of thousands of troops near Ukraine's borders in what Kyiv and its allies fear could be preparation for a new military offensive against Ukraine.
Adding to the jitters, Russia moved additional troops into Belarus this week ahead of what Minsk said were planned joint exercises next month.
Moscow denies plans to launch an attack but has pressed Washington for security guarantees, including a block on Ukraine joining the NATO alliance.
Speaking to diplomats at the U.S. embassy in Kyiv, Blinken said he strongly hopes that Russia can stick to a diplomatic and peaceful path when he meets Lavrov, and warned that Russian President Vladimir Putin could give the order to attack at short notice.
"As you all know very, very well, we have been engaged in the past couple of months in an intense focus on Ukraine because of the significant buildup we've seen of Russian forces we’ve seen near the Ukrainian border," Blinken said.
The Russian buildup, he said, was taking place with "no provocation, no reason."
"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.