US Secretary of State Antony Blinken says the US could increase security assistance for Ukraine after what he called Russia's "reckless and aggressive" actions in massing troops near its border.
During a visit to Kyiv intended to show support for Ukraine, Blinken said Russia had left behind significant numbers of troops and equipment despite announcing a withdrawal of its forces from close to the border after a stand-off that alarmed the European Union.
Blinken also said US President Joe Biden was keen to visit Ukraine and meet President Volodymyr Zelenskiy but gave no details on that or on Ukraine's aspirations to join the NATO military alliance.
"We are aware that Russia has withdrawn some forces from the border of Ukraine but we also see that significant forces remain there, significant equipment remains there," Blinken said, speaking alongside Zelenskiy.
"We are monitoring the situation very, very closely," he said.
"And I can tell you, Mr President, that we stand strongly with you, partners do as well. I heard the same thing when I was at NATO a couple of weeks ago and we look to Russia to cease reckless and aggressive actions."
The US is "actively looking at strengthening even further our security co-operation and our security assistance," he said without giving details.
Zelenskiy said Russia had withdrawn only about 3500 of the tens of thousands of troops deployed to the Crimea peninsula which it annexed from Ukraine in 2014.
"There may be a threat. Nobody wants these surprises," he said.
Blinken mixed solidarity with calls for Ukraine to stick to a path of reforms and fighting corruption and the influence of oligarchs.
The State Department expressed concern about the firing of a reformist energy official last week.
Biden had pledged "unwavering support" to Zelenskiy in April as officials in Kyiv and Moscow traded blame for clashes in Ukraine's eastern Donbass region and Russia's troop deployment.
Russia announced a withdrawal of its forces on April 22, helping pave the way for a summit between Biden and Russian President Vladimir Putin, possibly as early as June.
The stand-off prompted Ukraine to call for the US and European countries to help accelerate its NATO entry.
Zelenskiy asked Blinken for support in securing a Membership Action Plan at a NATO summit in June.
Australian Associated Press