US President Joe Biden warned President Vladimir Putin in a tense, two hour virtual summit Tuesday of a "strong" Western economic response should Russian forces massed on Ukraine's border go on the attack.

"President Biden voiced the deep concerns of the United States and our European allies about Russia's escalation of forces surrounding Ukraine and made clear that the US and our allies would respond with strong economic and other measures in the event of military escalation," the White House said in a statement soon after the video conference

Biden stressed "support for Ukraine's sovereignty and territorial integrity and called for de-escalation and a return to diplomacy," the statement said.

According to the White House, Biden and Putin agreed that their respective teams would "follow up" on the summit, underlining that the next US move would be "in close coordination with allies and partners."

Russia denies planning to invade Ukraine, but with satellite pictures showing massive troop concentrations on the border, fears are growing of war in Europe.

Reflecting the brittle atmosphere, Biden sat behind closed doors in the White House's high security Situation Room. By contrast, Biden held a similar video summit with Chinese President Xi Jinping three weeks ago in the more decorative Roosevelt Room, with journalists invited to witness the opening minutes.

The United States says it doesn't know for sure what Russia intends to do in Ukraine, but is alarmed at the deployment of some 100,000 battle-ready Russian troops to the border. Russia already supports a powerful separatist rebellion across swaths of eastern Ukraine and annexed the Crimean peninsula from Kiev in 2014.

Moscow calls invasion talk "hysteria." Instead, Putin intended to tell Biden he sees Ukraine's growing alliance with Western nations as a threat to Russian security -- and that any move by Ukraine to join NATO or to host NATO missiles would be unacceptable.

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

"Russia has never planned to attack anyone," Kremlin spokesman Dmitry Peskov said Tuesday. "But we have our own red lines."

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

US, Europe coordinate approach

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 Western appetite for sending US or European troops into direct military conflict with Russia, leaving limited options for pressuring Moscow.

Biden talked with the leaders of Britain, France, Germany and Italy on Monday to coordinate his message ahead of the summit, and the White House said he would speak again to the four countries after he was finished with Putin.

"The leaders agreed to stay in close touch on a coordinated and comprehensive approach in response to Russia's military build-up on Ukraine's borders," the White House said.

Biden was also planning to reach out to Ukraine's President Volodymyr Zelensky after his virtual summit.

'Severe harm'

The United States says it could impose painful economic sanctions, but has yet to go deep into specifics.

"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 Monday.

Beyond Ukraine, Biden and Putin also discussed a range of issues where their countries are at odds, including Iran's problematic nuclear program and a wave of cyberattacks 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.