In case Russia attacked Ukraine, a senior White House official said, the United States was willing to "respond" to potential requests for an increase in its military presence.

"If Russian President Vladimir Putin moves, there will be a request from our allies" in Eastern Europe "to send more troops and capabilities, and the United States will respond."

But the senior official explained that a direct US military response in the event that Ukraine was attacked is not here.

"The United States does not seek to find itself in a situation where the direct use of American troops is on the table."

He noted that in the event of an invasion, Washington would prefer to work on a "combination of several elements," including "support to the Ukrainian military," "strong economic sanctions," and "a substantial increase in the support and capacity of our NATO allies."

US President Joe Biden will make contact with Ukrainian President Volodymyr Zelensky "in the days following" his talks with his Russian counterpart Vladimir Putin, as confirmed by the White House official, explaining that Foreign Minister Anthony Plinkin will speak to the Ukrainian President.

The same official added that the US President would "hold talks later with key European allies to coordinate the message" he would send to his Russian counterpart in their video talks and to emphasize that he would "engage in these talks with Putin in unity with the allies and in strong cross-border solidarity."

The Kremlin has confirmed that no "breakthrough" is expected during the talks between Presidents Vladimir Putin and Joe Biden.

The two men who met in June in Geneva and made a phone call in early July are expected to focus on the Ukrainian issue.

Kiev and its allies accuse Russia of mobilizing troops and armour at its borders in preparation for an attack, and Moscow in return denies any intention of waging war.