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Melinda Simmons also described the harrowing scenes she had witnessed in towns outside the capital, which experienced intense fighting, with hundreds of civilians killed.
She said that despite the Russians being pushed out of the Kyiv region last month, the capital still felt “unsafe” with the pervasive threat of invasion.
Ms Simmons, who took up her post in 2019, spoke to the foreign press after a two-week trip to Ukraine as she seeks to set up a full-time ambassadorial presence, despite concerns of Russia reinvading.
“Nobody thinks that Kyiv is out of scope,” she said, speaking in London. “Everybody thinks that President Putin wants to have another go at Kyiv. There is, of course, potentially a big gap between what he wants to do and what he's able to do.”
Air raid sirens still sound at night and in the afternoons, although there was some reassurance from the capital’s widespread air defence system.
“Kyiv feels very well secured but it does not feel safe, it feels quite watchful and that posture is probably right,” Ms Simmons said.
She said people still had to be on their guard because “the moment that Ukrainians start to look like they are relaxing and going back to normal is the moment that Putin will take as a sign of weakness and when he will strike”.
The capital city was still marked with military checkpoints and sandbags, she said, but the numbers returning far outnumbered those leaving, although much of the infrastructure has been fractured by Russian bombing.
“Everybody is watchful of where they need to be and many people I know who have returned to Kyiv have not returned to their apartments if they live in high-rise blocks,” she said.
She also issued a warning that the “strategic objective of taking the whole of Ukraine remains in Putin's mind”, not only the eastern part. “To take the south and then move north remains in people's minds on what Putin wants,” she said.
The ambassador also visited the devastated town of Irpin, where she saw the destructive aftermath of the invasion.
“Having seen civilian cars abandoned all over the city riddled with bullets and bloodstains on the passenger seats, which is hard for anyone to see, shows that civilians were being shot at as they were driving to get away,” said Ms Simmons, who is of Polish and Ukrainian heritage.
Much of Irpin was left in ruins, including the Museum of Ukrainian Heritage, which suggested that the Russians had deliberately targeted the institution. “That really struck me ... how far there is a deliberate attempt to deny, subvert and even destroy Ukrainian culture, language and heritage,” Ms Simmons said.
She added it was “incredibly upsetting” that elderly residents were still in shelters in freezing conditions without electricity.
“They're terrified of what's happening outside, even with reassurances that it has been calmer than it has been for a while,” she said.
Despite the difficult fighting in eastern Ukraine, its people — from civilians to those in government — were adamant they would continue battling, as it was an “existential story” for Ukraine.
“Most of them don't think there's a choice, you have to fight on," Ms Simmons said. "If you stop fighting, you lose the country."