"We're now seeing the largest concentration of Russian forces on Ukraine's border since 2014, and that is of deep concern not only to Ukraine, but to the United States, and indeed to many of our allies and partners", Mr Blinken said as he met with Mr Kuleba in the Belgian capital.
The Kremlin said on Tuesday that it dismayed over Turkish-made Bayraktar TB2 drone systems in the Donbass region of Ukraine.
Amid the rising tensions, the United States has notified Turkey that two US warships will sail to the Black Sea on April 14 and April 15 and stay there until May 4 and May 5. Provocative in the direct sense of the word: they are testing our strength, playing on our nerves.
Blinken's schedule Tuesday includes talks with Ukrainian Foreign Minister Dmytro Kuleba. "That's particularly important at a time when were seeing, unfortunately, Russian Federation take very provocative actions when it comes to Ukraine".
"We call on Russian Federation to cease its provocations and to immediately de-escalate tensions in line with its worldwide obligations".
Ukraine's leader has asked for a phone call with Russian President Vladimir Putin about the Russian troop buildup across his country's border and the escalating tensions in eastern Ukraine, but the request has not been answered so far, his spokeswoman said Monday.
Kramatorsk border units affiliated with Ukrainian Armed Forces' Joint Forces Border Units patrol seen on April 12.
"Russia must end this military build-up in and around Ukraine, stop its provocations and de-escalate immediately".
Russian Defence Minister Sergei Shoigu said on Tuesday Russia had moved two armies and three paratrooper units to near its western borders in the last three weeks, responding to what it called threatening military action by North Atlantic Treaty Organisation. These numbers are likely to grow as the troops "keep arriving", she said.
Kuleba said Kyiv wanted a diplomatic solution.
Local citizens visit their home in the separatist-controlled territory to collect belongings after a recent shelling near a frontline outside Donetsk, eastern Ukraine, Friday, April 9, 2021.Tensions have built up in recent weeks in the area of the separatist conflict in eastern Ukraine, with violations of a cease-fire becoming increasingly frequent.
Ukraine - which applied to join the US-backed alliance in 2008 - is pressing Western powers for "practical" support as it seeks to deter any new aggression from Moscow. On Monday, Ukraine's military reported one more serviceman killed, bringing the total to 28 this year.
Putin similarly held a summit in Finland in 2018 with then US president Donald Trump, who caused a furor at home by appearing to accept the Russian leader's denials of election meddling.
Joining Blinken in Brussels is U.S. Defense Secretary Lloyd Austin.
Crimea is still recognized as part of Ukraine by the United Nations, and Kiev vows to one day win it back.