U.S. Secretary of State Antony Blinken said on Wednesday 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 Kyiv to show support for Ukraine, the top U.S. diplomat said Ukrainians should prepare for difficult days. He said Washington would keep providing defense assistance to Kyiv 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 U.S. response to its sweeping demands for security guarantees from the West. Russia has demanded NATO stops its eastward expansion and return its force deployments to the positions they held in May 1997.

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'Train wreck' coming

The pessimistic statements highlighted the gulf between Washington and Moscow as 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."

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 the Ukrainian border 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.

Troop movements

Russia has also moved troops to Belarus for what it calls joint military exercises, giving it the option of attacking neighboring 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. 

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.

Dire assessment

The U.S. says Russia is threatening its post-Soviet neighbor 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 Kyiv'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. Washington says these demands are "non-starters."

Vladimir Frolov, a former Russian diplomat who is now a foreign policy analyst, said Moscow would not be appeased by a U.S. 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 U.S. surrender and their delivering Ukraine to Russia, some kind of a military option is all but inevitable now."

Aid for Ukraine

Blinken said he would not be presenting a written response to Lavrov in Geneva on Friday – something Russia has repeatedly demanded.

He said both sides needed to take stock of a series of diplomatic meetings on the crisis last week.

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

President Joe Biden's administration last month approved the provision of an additional $200 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. Canada has deployed special forces in the country and says it may supply hardware.

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.