Russia denies any plans to move against Ukraine, but with satellite pictures showing massive troop concentrations on the border, fears are growing over war in Europe.

Biden and Putin, who last met in Geneva in June, were to begin their video summit at 1500 GMT -- 10:00 am in Washington and 6:00 pm in Moscow. Unlike a similar video link between Biden and Chinese President Xi Jinping three weeks ago, the Putin call was taking place entirely behind closed doors.

The United States says it doesn't know what Russia intends to do in Ukraine, but has raised alarm over movements of what US officials say are some 100,000 battle ready troops.

Russia, which calls invasion talk "hysteria," already supports a powerful separatist rebellion controlling swaths of eastern Ukraine and annexed the Crimean peninsula from Ukraine in 2014.

Putin intends to tell Biden of concerns that Ukraine's growing alliance with Western nations threatens Russian security -- and that any move by Ukraine to join NATO would be crossing a red line.

Even though Ukraine is nowhere near being able to join the US-led military alliance, Putin wants a "legal" guarantee this will never happen.

"Our president is ready to convey his concerns to his American colleague, listen to his concerns and give appropriate explanations," Kremlin spokesman Dmitry Peskov said Tuesday.

"Russia has never planned to attack anyone," he said. "But we have our own red lines."

The United States and NATO say Russia cannot be given a veto over Ukraine's ambitions.

- 'Real cost' -

The United States and European allies have worked to bolster Ukraine's military since 2014 when the country's armed forces crumpled in the face of Russian pressure. However, there is no appetite for direct military conflict with Russia.

The White House says Biden will instead lay out a range of economic sanctions that will impose "real cost."

Ahead of the summit, the Biden administration has also stressed that its posture has been worked out in unison with European partners.

"We've consulted significantly with our allies and believe we have a path forward that would impose significant and severe harm on the Russian economy," White House Press Secretary Jen Psaki told reporters.

"You can call that a threat. You can call that a fact," she said.

And while there is no expectation for a US troop deployment to Ukraine itself, a Russian invasion would spur boosts in NATO strength in other parts of eastern Europe that are part of the alliance, the White House says.

"If Putin moved in, there would be an increasing request from eastern flank allies and a positive response from the United States for additional forces," a senior US official told reporters.

On the eve of his virtual meeting, Biden spoke with the leaders of France, Germany, Italy and Britain, with the Western powers expressing their "determination" that Ukraine's sovereignty be respected.

The White House said that after the talk with Putin, Biden would reach out to Ukraine's President Volodymyr Zelensky.

Beyond Ukraine, Biden and Putin will also discuss a range of big issues where their countries are at odds, including Iran's problematic nuclear industry and a wave of cyber attacks against the United States.

- Zelensky to the front -

Clad in a combat uniform, Zelensky visited troops fighting pro-Moscow separatists in the country's east on Monday.

The conflict has claimed over 13,000 lives and while Ukrainian forces are currently in a deadlock against their separatist opponents, they would likely be overwhelmed if Russian regular troops crossed the border.

"Thank you for protecting the sovereignty and territorial integrity of Ukraine," Zelensky told the soldiers, according to a statement released by Kiev.

On Monday, he spoke with US Secretary of State Antony Blinken, writing on Twitter after that he was "grateful" to the United States and allies for supporting Ukraine.

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