US Secretary of State Antony Blinken urged Russia to take a “peaceful path” on Ukraine as he arrived in Kiev in a show of support.
Antony Blinken arrived in Europe on Wednesday for four-way talks with Britain, France and Germany in Berlin and a showdown with Russian Foreign Minister Sergey Lavrov on Friday in Geneva.
He promised "relentless diplomatic efforts to prevent renewed aggression and to promote dialogue and peace" in the region.
His comments, however, risked being overshadowed by subsequent remarks from US President Joe Biden, who during a lengthy press conference appeared to suggest that a “minor incursion” would be met with a lesser US response.
“Russia will be held accountable if it invades — and it depends on what it does. It's one thing if it's a minor incursion and we end up having to fight about what to do and what to not do, et cetera,” Mr Biden said at a press conference.
White House Press Secretary Jen Psaki moved quickly to clarify the remarks, saying Mr Biden "has been clear with the Russian president: If any Russian military forces move across the Ukrainian border, that's a renewed invasion, and it will be met with a swift, severe, and united response from the United States and our allies."
About 100,000 Russian troops are massed on Russia's border with Ukraine, leading to fears in the West of an imminent invasion, claims that Moscow rejects.
"I strongly, strongly hope that we can keep this on a diplomatic and peaceful path but ultimately that's going to be President Putin's decision," Mr Blinken said.
"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."
Ukrainian President Volodymyr Zelenskiy thanked Mr Blinken for an increase in US support, believed to be an additional $200 million in defensive military aid.
"I would like to thank you personally, and President Biden and the US administration for your support, for military assistance to Ukraine, for increasing this assistance," Mr Zelenskiy told Mr Blinken.
At a press conference with his Ukrainian counterpart Dmytro Kuleba, Mr Blinken said the US was ready to up its "defensive assistance" to Ukraine if Russia increased its aggression.
"Should Russia carry through with any aggressive intent, and renew its aggression and invade Ukraine, we'll provide additional material beyond [what] is already in the pipeline, and that will further aid in defending Ukraine," Mr Blinken said.
Russia annexed the Crimean region in 2014 and backed separatists against Ukraine in a conflict that has swept over large areas in eastern Ukraine.
The Ukrainian president said his country had in recent years allocated the maximum amount of money possible to strengthen its armed forces.
"There are some 100,000 Russian soldiers near Ukraine's borders and in that sense the threat to Ukraine is unprecedented," Mr Blinken said.
"So the president [Joe Biden] asked me to underscore once again our commitment to Ukraine's territorial integrity, to its sovereignty, to its independence.
"But all the same, we understand that to take steps quickly to modernise the army, we need help, especially in these times, in difficult times."
Russia has put forward a string of demands to Nato to help ease tension over Ukraine, including a ban on Kiev joining the military alliance. Nato rejected that request out of hand.
Nato-Russia talks were held last week on the proposals with little progress made. Mr Lavrov said there would be no continuation of the negotiations until Nato responded to Russia’s demands.
These include measures that would limit military activity in former Warsaw Pact and former Soviet countries that joined the alliance after the Cold War.
Russia's deputy foreign minister, Sergei Ryabkov, who led Russia's delegation at the security talks with the US in Geneva last week, underlined that his country had no intention to invade Ukraine.
But he said Russia's security demands “constitute a package, and we’re not prepared to divide it into different parts, to start processing some of those at expense of standing idle on others”.
Referring to Nato powers giving Kiev military assistance, he said: "We see the threat of Ukraine becoming ever more integrated in Nato without even acquiring a formal status of a Nato member state.
“This is something that goes right to the centre of Russia’s national security interests, and we will do our utmost to reverse this situation, to rebalance this situation through diplomatic means.”
Nato member states have warned Russia it will face severe economic measures if it does invade Ukraine.
French President Emmanuel Macron, addressing the European parliament on Wednesday, called for "frank and demanding" talks with Russia.