President Joe Biden and his Ukrainian counterpart Volodymyr Zelensky will speak by phone on Sunday, both sides confirmed, after the US leader again warned that Russia's Vladimir Putin would face a tough response to any invasion of the eastern European country.

Using some of his most direct language yet, Biden said Friday, "I'm not going to negotiate here in public, but we made it clear he cannot -- I’ll emphasize, cannot -- invade Ukraine."

He added, in brief remarks to reporters in Wilmington, Delaware, that he had "made it clear to President Putin that we will have severe sanctions, we will increase our presence in Europe, with NATO allies" if Russia invades Ukraine.

During the call Sunday with Zelensky, a White House official said, Biden will "reaffirm US support for Ukraine's sovereignty and territorial integrity, discuss Russia's military build-up on Ukraine’s borders, and review preparations for upcoming diplomatic engagements to help de-escalate the situation in the region."

Zelensky tweeted: "Look forward to talking again with @POTUS this Sunday to coordinate our steps for the sake of peace in Ukraine and security in Europe."

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.

Moscow describes the troop presence as protection against the expansion of NATO, although Ukraine has not been offered membership in the military alliance.

US Secretary of State Antony Blinken spoke Friday with NATO chief Jens Stoltenberg; afterward, Blinken urged Russia to "engage meaningfully" with upcoming talks on the tense standoff between Moscow and Western-backed Ukraine.

Stoltenberg said that NATO was "united" and "prepared for dialogue."

The latest diplomatic push came one day after Biden on Thursday warned Putin against invading 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.

Putin was "pleased" overall with the talks, foreign policy adviser Yury Ushakov told reporters. 

A senior US official, who spoke on condition of anonymity, said the tone "was serious and substantive."

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.

- US warns of tough sanctions -

In a readout after Thursday's 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.

The January talks will see Russian officials sitting down separately with negotiators representing the United States, NATO and the regional OSCE security forum, which also includes Washington.

Ukraine, which wants to join NATO but has been told it is far from being ready to win acceptance, is eager not to be cut out of any wider deal.

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.