The United States has informed the Ukrainian government that its latest intelligence points to a full-scale imminent Russian attack and has warned that the northeastern city of Kharkiv is at "at particular risk," a CNN journalist reported in a tweet late Wednesday, citing Ukrainian and U.S. officials and confirming an earlier report by Newsweek.

There has been "a big build-up" of Russian armor and support vehicles less than 30 kilometers (18 miles) across the border from Kharkiv in Russia, Katie Bo Lillis tweeted. Ukraine says it does not plan to evacuate Kharkiv, however, Ukrainian President Volodymyr Zelenskyy has called for a state of emergency across the country starting midnight, Lillis added.

"A senior Ukrainian official points out to CNN the U.S. has issued similar warnings before for assaults that did not materialize," the journalist noted.

Violence has recently spiked again in Ukraine's east, where an eight-year conflict between Russia-backed rebels and Ukrainian forces has killed nearly 14,000 people. The Ukrainian military said that one Ukrainian soldier was killed and six more sustained injuries after shelling by the rebels. Separatist officials reported several explosions on their territory overnight and three civilian deaths.

Since last Friday, when separatist leaders in the Donetsk and Luhansk regions announced mass evacuations into Russia, more than 96,000 residents of the separatist areas have crossed the Russian border.

After weeks of rising tensions, Putin took a series of steps this week that dramatically raised the stakes. First, he recognized the independence of those separatist regions. Then, he said that recognition extends even to the large parts of the territories now held by Ukrainian forces, including the major Azov Sea port of Mariupol.

Finally, he asked for and was granted permission to use military force outside the country – effectively formalizing a Russian military deployment to the rebel regions.

Still, Putin suggested there was a way out of the crisis, laying out three conditions: He called on Kyiv to recognize Russia's sovereignty over Crimea, the Black Sea peninsula that Moscow annexed from Ukraine in 2014, to renounce its bid to join NATO and partially demilitarize.