KIEV (REUTERS, AFP) - US Secretary of State Antony Blinken said on Wednesday (Jan 19) that Russia could launch a new attack on Ukraine at very short notice, but Washington would pursue diplomacy as long as it could.

On a visit to Kiev to show support for Ukraine, the top US diplomat said Ukrainians should prepare for difficult days.

He said Washington would keep providing defence assistance to Kiev 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.

Blinken said on Wednesday that he will not present a formal response to the Russian proposals on the Ukraine crisis in talks this week, saying the two sides needed to explore common ground.

Russia - whose foreign minister Sergei Lavrov will meet Blinken in Geneva on Friday - presented unusually detailed draft proposals to the United States after amassing tens of thousands of troops on Ukraine's borders.

"I won't be presenting a paper at that time to Foreign Minister Lavrov," Blinken told reporters in Kiev. "We need to see where we are and see if there remain opportunities to pursue the diplomacy and pursue the dialogue which, as I have said, is by far the preferable course."

Blinken reiterated that some Russian ideas were "clearly, absolutely, non-starters" such as explicitly barring Ukraine from joining Nato.

Instead of making counter-proposals, "we have raised our concern about challenges that Russia poses to the security of the European area", Blinken said.

The meeting comes the week after the two top diplomats' deputies met in Geneva and the US proposed working together with Russia on arms control. "We talked about areas where clearly, if there is a will, we could make progress on a reciprocal basis to improve security for everyone," Blinken said.

Blinken promised "relentless diplomatic efforts to prevent renewed aggression and to promote dialogue and peace".

He said the Russian build-up of troops 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.

He did not spell out how quickly Russia might move.

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.

Moscow 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 tension around 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.

'Hopes are dim'

The United States says Russia is threatening its post-Soviet neighbour and may be poised for a new invasion, 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 Kiev's growing ties with the West. It wants to impose "red lines" to prevent Ukraine ever joining Nato and to get the alliance to pull back troops and weapons from eastern Europe.

Vladimir Frolov, a former Russian diplomat who is now a foreign policy analyst, said Moscow would not be appeased by a US and Nato offer of arms control talks and was pursuing a much more sweeping rearrangement of the European security order.

"The Lavrov-Blinken meet is probably the last stop before the train wreck. But hopes are dim, the positions are incompatible," he said.

Describing Russia's military deployment in Belarus as a "huge escalation", 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."

Ukrainian President Volodymyr Zelenskiy told Blinken in Kiev on Wednesday: "I would like to thank you personally and President Biden and the US administration for military support for Ukraine, for increasing this assistance."

President Joe Biden's administration last month approved the provision of an additional US$200 million (S$270 million) in defensive security assistance to Ukraine and gave more such aid last year than at any point since Russia's annexation of Crimea in 2014.

On Monday, Britain said it had begun supplying Ukraine with anti-tank weapons to help it defend itself.

Russian Deputy Foreign Minister Sergei Ryabkov called on the West on Wednesday to stop supplying Ukraine with weapons and described the situation around European security as "critical", the Interfax news agency reported.