However, Russian officials, who over the weekend held their own talks with the U.S. team in the Sunshine State, continued to downplay any sense of progress and Moscow still shows little sign of yielding from the maximalist war demands that Ukraine surrender territory, reduce its army and never join NATO.

Russia launched a "massive" attack on several Ukrainian cities overnight on Monday, President Volodymyr Zelensky has said, a day after he warned of strikes over the Christmas period.