US President Joe Biden wants to further arm Ukraine in the event of an invasion by Russia, according to the White House.
"We would provide additional defensive materiel to the Ukrainians above and beyond that which we are already providing," Biden's National Security Adviser Jake Sullivan said after a video summit between the US president and his Russian counterpart Vladimir Putin on Tuesday.
"And we would fortify our NATO allies on the eastern flank with additional capabilities in response to such an escalation," Sullivan added.
Biden again warned Putin of economic consequences in the event of an escalation in the conflict with Ukraine.
The White House said after the conversation that Biden expressed "deep concerns of the United States and [its] European allies about Russia's escalation of forces surrounding Ukraine."
The summit, which focused on tensions over the Ukraine conflict, was scheduled to run for several hours, but ended after only about two hours.
The Kremlin had previously stated that it could become a very long conversation in view of the multitude of conflict topics.
Biden made it clear that "if Russia further invades Ukraine, the United States and [its] European allies would respond with strong economic measures," according to Sullivan.
Biden, however, reiterated that there is another option: "de-escalation and diplomacy."
"President Biden was direct and straightforward with President Putin as he always is, and reiterated America's support for Ukraine sovereignty and territorial integrity," Sullivan said.
Sullivan said Biden would speak with Ukrainian President Volodymyr Zelensky on Thursday.
'A bloody massacre'
In an interview with US news channel CNN, Ukrainian Defence Minister Oleksiy Reznikov warned that a Russian invasion of Ukraine would mean a "really bloody massacre."
Reznikov warned that Russians would "come back in the coffins."
After his video conference with Putin, Biden called and briefed European allies.
"President Biden briefed leaders on his call with President Putin, in which he discussed the serious consequences of Russian military action in Ukraine and the need to de-escalate and return to diplomacy," the White House said in a statement.
He spoke with outgoing German Chancellor Angela Merkel, French President Emmanuel Macron, British Prime Minister Boris Johnson and Italian Prime Minister Mario Draghi.
"The leaders underscored their support for Ukraine's sovereignty and territorial integrity, as well as the need for Russia to reduce tensions and engage in diplomacy. They agreed their teams will stay in close touch, including in consultation with NATO allies and EU partners, on a coordinated and comprehensive approach," the White House added.
Earlier, incoming German chancellor Olaf Scholz expressed concern about the Russian troop deployment on the border with Ukraine.
Security and cooperation in Europe presuppose principles that were negotiated in the policy of detente and continue to have an effect today, Scholz warned.
"These include the inviolability of borders," said Scholz, who is due to be officially elected Germany's new chancellor on Wednesday.
For weeks, the US has been accusing Russia of deploying troops not far from the border with Ukraine. The West fears a Russian invasion of the former Soviet republic.
Russia rejects this and accuses Ukraine of having moved more than 120,000 soldiers to the front line with the separatist regions.
Putin reiterated his demand for a halt to NATO's eastward expansion during the talks.
Russia wants binding legal guarantees that the Western military alliance will not expand eastwards and station offensive weapons there, Putin said in a statement released by the Kremlin late on Tuesday evening.
Russia sees itself threatened by a NATO advance and wants to prevent neighbouring former Soviet republics Ukraine and Georgia from joining the alliance.
Putin had also said that moving NATO military infrastructure into Ukraine would be crossing a "red line" from Russia's point of view.
No "breakthroughs" were expected from the summit, Kremlin spokesman Dmitry Peskov stressed. He called the meeting a working discussion at a very difficult time.
According to the Kremlin, the two heads of state connected via a specially protected and tap-proof line.
As heads of state, Putin and Biden had first met in person in Geneva in June.
According to the Kremlin, Putin and Biden also spoke about the cybersecurity of their countries after numerous hacker attacks. Talks on this should also be continued. The Kremlin confirmed that the Ukraine conflict had taken up most of the talks.