Biden and Putin trade warnings over Ukraine, but vow diplomacy

Ukrainian Territorial Defense Forces, the military reserve of the Ukrainian Armes Forces, train to counter a feared Russian invasion

President Joe Biden on Thursday warned his Russian counterpart Vladimir Putin of a tough US response to any invasion of Ukraine, while the Kremlin leader said anti-Moscow sanctions would be a "colossal mistake."

After a 50 minute phone call -- their second in just over three weeks -- both presidents indicated support for further diplomacy on the tense standoff between Russia and Western-backed Ukraine.

But there was no disguising the depth of disagreement -- or the dangerously high stakes on the fringes of eastern Europe -- ahead of in-person negotiations between high-ranking Russian and US officials on January 10.

Ushakov, referring to Washington's repeated threats of economic sanctions as a response to a Ukraine attack, said this would be "a colossal mistake. We hope this will not happen."

"President Biden reiterated that substantive progress in these dialogues can occur only in an environment of de-escalation rather than escalation," Psaki said.

Washington and its European allies accuse Russia of threatening former Soviet territory Ukraine with a new invasion. Some 100,000 Russian troops are massed near the border of the country, where Putin already seized the Crimea region in 2014 and is accused of fomenting a pro-Russian separatist war which erupted that same year in the east.

Earlier this month, the Russians issued a sweeping set of demands, including guarantees that NATO not expand and a bar on new US military bases in former territories of the Soviet Union.

In a readout after the call, the Kremlin stressed that Biden told Putin that US offensive weapons would not be deployed in Ukraine. The White House, however, said Biden merely reaffirmed existing policy.

And US officials repeated warnings of blistering economic sanctions backed both by Washington and EU capitals if Russia does attack Ukraine further.

- US support for Ukraine -

Russia's delegation will be led by Deputy Foreign Minister Sergei Ryabkov, and the US delegation by Deputy Secretary of State Wendy Sherman.

US officials have been at pains to insist that no decision will be taken behind the Ukrainians' backs and that while US troops would not be sent to defend the country against Russia, ongoing deliveries of weapons and other military assistance are set to expand if Moscow attacks.

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