US top envoy Antony Blinken says Washington would keep providing defence assistance to Kiev after a meeting with Ukrainian President Volodymyr Zelenskyy.

Blinken says a Russian buildup of tens of thousands of troops near the Ukrainian border is taking place with
Blinken says a Russian buildup of tens of thousands of troops near the Ukrainian border is taking place with "no provocation, no reason." (AP)

The United States has said 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, US Secretary of State Antony Blinken said on Wednesday that 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.

The pessimistic statements highlighted the gulf between Washington and Moscow as Blinken gears up for a meeting with Russian Foreign Minister Sergey 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 buildup 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 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.

READ MORE: US announces fresh military aid to Ukraine amid Russia invasion fears

Kremlin denies any intention of invasion

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 spokesperson Dmitry Peskov said Western weapons deliveries to Ukraine, military manoeuvres and NATO aircraft flights were to blame for rising tension around Ukraine.

READ MORE: Blinken urges Putin from Kiev to choose 'peaceful path'

Russian 'red lines'

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. 

Washington says these demands are "nonstarters".

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.

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

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

READ MORE: UK sends anti-tank weapons to Ukraine

READ MORE: Ukraine: Russia amasses 127,000 troops in 'almost complete' buildup

Source: TRTWorld and agencies