YIV, Ukraine — As Russia’s invasion of Ukraine nears the one-year mark, Kyiv is sounding an increasingly urgent warning: Vladimir Putin is preparing a major new offensive.

Ukrainian officials say they fear that Russia's military is regrouping and preparing an imminent attack designed to turn the tide of the war in Moscow's favor — its ranks bolstered by hundreds of thousands of conscripts called up last fall.

“We should understand that the threat of a new and another offensive will remain until we defeat Russia,” Yuriy Sak, a senior defense ministry official, told NBC News in an interview Thursday.

A spring offensive has long been predicted by Western officials and analysts, with the Kremlin eager to seize the initiative after a grinding winter that was preceded by months of battlefield setbacks and domestic criticism.

But leaders in Kyiv now say that Russia has amassed a large force ready to attack soon, in the run-up to the Feb. 24 anniversary.

Speaking to French media on Wednesday, Ukrainian defense minister Oleksii Reznikov said Russia had 500,000 troops ready for an assault in the coming weeks, nearly double the number Putin announced he was mobilizing in September.

“Officially, they announced 300,000, but when we see the troops at the borders, according to our assessments it is much more,” he said.