Xi Jinping has joined Joe Biden in condemning Russian threats of using nuclear weapons in Ukraine, the White House said, as the leaders of the USA and China met on Monday on the sidelines of the G20 gathering in Bali.The leaders of the two superpowers agreed that “a nuclear war should never be fought” and cannot be won, “and underscored their opposition to the use or threat of use of nuclear weapons in Ukraine,” according to US officials.