Russia could launch a new attack on Ukraine at "very short notice" but Washington would pursue diplomacy as long as it could, US Secretary of State Antony Blinken said.
The top US diplomat said Ukrainians should prepare for difficult days on a visit to Kyiv to show support for the country.
He said Washington would keep providing defence assistance to Ukraine and renewed a promise of severe sanctions against Russia in the event of a new invasion.
The Kremlin said tension around Ukraine was increasing and it was still waiting for a written US response to its sweeping demands for security guarantees from the West.
The pessimistic statements highlighted the gulf between Washington and Moscow as Mr Blinken gears up for a meeting with Russian Foreign Minister Sergei Lavrov on Friday that a Russian foreign policy analyst called "probably the last stop before the train wreck".
Mr Blinken promised "relentless diplomatic efforts to prevent renewed aggression and to promote dialogue and peace".
He said a Russian build-up of tens of thousands of troops near Ukraine's borders 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 (Vladimir) Putin the capacity, also on very short notice, to take further aggressive action against Ukraine," he said.
A convoy of Russian armoured vehicles moves along a highway in Crimea. Source: AAP
'No plans to attack, strike or invade'
He did not spell out how quickly Russia might move, and independent security analysts say they do not believe Moscow has so far assembled the logistics and medical units it would need to launch an immediate attack.
Russia has also moved troops to Belarus for what it calls joint military exercises, giving it the option of attacking neighbouring Ukraine from the north, east and south.
It continues to deny any such intention.
Kremlin spokesman Dmitry Peskov said Western weapons deliveries to Ukraine, military manoeuvres and NATO aircraft flights were to blame for rising tensions around Ukraine.
Russian Deputy Foreign Minister Sergei Ryabkov said he did not believe there was a risk of a large-scale war, reiterating Moscow had no plans to attack, strike or invade Ukraine.
Ukrainian Foreign Minister Dmytro Kuleba accused Russia of trying to sow panic in Ukraine. He said diplomacy offered the only way out, and it was an "indestructible principle" that no decisions about Ukraine could be taken without its involvement.
"The basic principle is simple: a strong Ukraine is the best instrument to restrain Russia," he said.
The United States said Russia may be poised for a new invasion of Ukraine, eight years after it seized Crimea and backed separatist forces who took control of large parts of eastern Ukraine.
Russia says it feels menaced by Kyiv's growing ties with the West, and wants to prevent Ukraine from ever joining NATO and for the alliance to pull back troops and weapons from eastern Europe.
"It's not clear what Russia's central demand is or is not. They've put a number of things on the table," Mr Blinken said.
"Some of them are clearly absolute non-starters like closing NATO's door to new members," he said, adding that Washington remained ready to discuss anything that enhanced "everyone's security on a reciprocal basis".
This could include arms control, risk reduction, and the scale and scope of military exercises, he said.
A Ukrainian soldier walks at the line of separation from pro-Russian rebels near Popasna in the Donetsk region, Ukraine. Source: AAP
Military option 'all but inevitable'
Vladimir Frolov, a former Russian diplomat who is now a foreign policy analyst, said Moscow would not be appeased by the US and NATO offer of arms control talks and was seeking a much more sweeping rearrangement of Europe's security order.
"The Lavrov-Blinken meet is probably the last stop before the train wreck. But hopes are dim, the positions are incompatible," said Mr Frolov.
Describing Russia's military deployment in Belarus as a "huge escalation", Mr Frolov gave a dire assessment of the crisis.
"I think barring a US surrender and their delivering Ukraine to Russia, some kind of a military option is all but inevitable now."