Blinken will meet Ukrainian President Volodymyr Zelenskiy and 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 Kiev 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 US embassy in Kiev, 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.
US President Joe Biden's administration last month approved the provision of an additional $US200 million in defensive security assistance to Ukraine and gave more such aid last year than at any point since Russia's annexation of Crimea from Ukraine in 2014.
Washington has warned Russia of severe consequences if it mounts a new offensive, while promising to beef up its security presence in Europe.
"Should Russia further invade Ukraine, we will provide additional defensive material to the Ukrainians above and beyond that which we are already in the process of providing," a State Department official said ahead of Blinken's arrival.
Blinken will meet Zelenskiy and Foreign Minister Dmytro Kuleba on Wednesday.
Then in Berlin he will meet German Foreign Minister Annalena Baerbock and later the Transatlantic Quad, referring to a format that involves the United States, Britain, France and Germany.
Germany signalled on Tuesday that it could halt the Nord Stream 2 pipeline from Russia if Moscow invades Ukraine.
Blinken spoke with Lavrov on Tuesday and the two decided in the call that it would be useful to meet in person.
Lavrov separately said Moscow would welcome US diplomatic efforts and reiterated Russian accusations that Ukraine was "sabotaging" agreements aimed at ending the conflict between Ukrainian troops and Russian-backed forces in eastern Ukraine.
Kiev has sought weapons from Western nations to shore up its defence. On Monday, Britain said it had begun supplying Ukraine with anti-tank weapons to help it defend itself.
