With tens of thousands of Russian troops massed on the Ukrainian border, tensions between Moscow and the West have reached a post-Cold War high and there are growing fears of a major conflict in Eastern Europe.
Moscow insists it has no plans to invade, but is demanding wide-ranging security guarantees -- including a ban on Ukraine ever joining NATO -- in exchange for deescalation.
Blinken's arrival Wednesday in Europe upped the diplomatic stakes, and after Kyiv he was headed to Berlin for four-way talks with Britain, France and Germany to seek Western unity, and finally to Geneva on Friday for a meeting with Russian Foreign Minister Sergei Lavrov.
New US military aid
Speaking at the embassy shortly after he arrived, Blinken warned that Russia could easily send more forces towards Ukraine.
"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.
A senior US official meanwhile confirmed that the United States had authorised an additional $200 million (176 million euros) in security aid to Ukraine, on top of $450 million already delivered by President Joe Biden's administration.
"We are committed to Ukraine's sovereignty and territorial integrity and will continue to provide Ukraine the support it needs," the official said on condition of anonymity.
Blinken later met with President Volodymyr Zelensky, who thanked Washington for its support, including the increased military assistance.
"We understand that to take steps quickly to modernise the army, we need help, especially in these... difficult times," Zelensky said.
Kyiv has been battling pro-Russian forces in two breakaway eastern regions since 2014, when Russia annexed the Crimean peninsula from Ukraine, with more than 13,000 people killed so far.
On the streets of the separatist-held city of Donetsk in eastern Ukraine, residents said they hoped a broader conflict could be avoided.
"Talks are good, at least it's not war," Alexei Bokarev, a 77-year-old retired miner, told AFP in the city, where signs on Soviet-era buildings read "Glory to the Warriors of Liberation".
"The guns are quiet and negotiations are going on, it means that there is some kind of a search for a solution. How will this end? Nobody can say," he said.
'Extremely dangerous situation'
As Blinken left Washington, White House Press Secretary Jen Psaki warned of an "extremely dangerous situation" around Ukraine, saying "no option is off the table" in terms of a US response to an invasion.
Lavrov on Tuesday said there would be no further negotiations until the West responds, in writing, to its demands for sweeping security guarantees.
As well as a permanent ban on Ukraine joining NATO, Moscow is demanding measures that would limit military activities in former Warsaw Pact and ex-Soviet countries that joined the alliance after the Cold War.
Washington has rejected the demands as "non-starters" and warned that any invasion of Ukraine would be met with severe economic counter-measures.
Kremlin spokesman Dmitry Peskov said Moscow was still waiting for written answers to its proposals, but described Friday's talks in Geneva as "extremely important".
Tensions increased on Tuesday with the launch of joint military drills between the forces of Russia and ex-Soviet Belarus, which also neighbours Ukraine.
A US official said the exercises could presage a permanent Russian military presence involving both conventional and nuclear forces in Belarus.