Rose Gottemoeller, who served as Nato’s deputy secretary general between 2016 and 2019, voiced her fears after Ukraine retook much of Kharkiv province in less than a week.
Speaking on BBC Radio 4’s Today programme, she praised Ukraine for its successes but cautioned that the Kremlin could respond in “unpredictable” ways.
“The momentum is clearly on the Ukrainian side at this moment,” Ms Gottemoeller said.
“If one looks at the maps, they’re just amazing; the splotch of colour showing the Ukrainian acquisitions over the past 48-to-72 hours is really quite remarkable.”
The former deputy Nato chief then warned that Moscow could resort to using weapons of mass destruction in an attempt to make Kyiv capitulate.
“I fear they will strike back now in really unpredictable ways and in ways that may even involve weapons of mass destruction,” she said.
Although she believes the Kremlin would not launch intercontinental ballistic missiles, the senior ex-diplomat said he could order a nuclear demonstration strike.
“He [Putin] might put in play a nuclear demonstration strike, either a single strike over the Black Sea or perhaps a strike at a Ukrainian military facility in order to strike terror not only into the hearts of the Ukrainians but also the global partners and allies of Ukraine.”
“Where we are now after this Ukraine success in the north is not that point,” he said. “But it is a lot closer to it than we’ve been before.”
Mr Bolton was Trump’s national security adviser between 2018 and 2019.
Their warnings come as Ukraine continues to make good progress in the northeast Kharkiv and the southern Kherson regions, according to Western military analysts.
On the road to the recaptured strategic city of Balakliia, Ukraine’s deputy defence minister Hanna Malya said on Tuesday: “The aim is to liberate the Kharkiv region and beyond - all the territories occupied by the Russian Federation.
“Fighting is continuing (in Kharkiv region). It is still early to say full (Ukrainian) control has been established over Kharkiv region,” she added.
Ukrainian president Volodymyr Zelensky has once more urged the West to speed up weapon deliveries, urging it to “strengthen cooperation to defeat Russian terror”.
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