Ukraine war live updates: Blinken visits Kyiv to boost morale as Ukrainian forces battle Russian onslaught in northeast border region

This is CNBC's live blog tracking developments on the war in Ukraine. See below for the latest updates. 

U.S. Secretary of State Antony Blinken is in Kyiv on Tuesday, with the visit designed to bolster Ukrainian morale as its forces battle a large-scale Russian offensive in the northeast border region of Kharkiv.

The visit to Kyiv, which was not disclosed beforehand, is the first by a top U.S. official since Congress passed a $61 billion military aid package for Ukraine in April.

Blinken's trip is meant to send a strong signal of reassurance to the Ukrainians during a "difficult moment" for the nation, a senior U.S. official told reporters as Blinken travelled by train from Poland to Kyiv.

"It's a tough fight. There is no question," said the official, who spoke on condition of anonymity, noting, however, that the U.S. has "a lot of confidence that the Ukrainians will increasingly be effective in pushing the Russians back as our assistance flows in both from the United States and other allies and partners."

Blinken's mission on this trip is to talk about how U.S. supplemental assistance will help shore up Ukraine's defenses, the official added, and to discuss other steps the U.S. is taking to shore up Ukraine's security.

U.S. Secretary of State Antony Blinken's surprise trip to Ukraine on Tuesday comes at what a senior U.S. official described as a "difficult moment" for Kyiv as its forces battle a new, large-scale Russian offensive in the northeast Kharkiv region.

Russia's Defense Ministry has already claimed that its forces have "liberated" a handful of settlements near the border but fighting is at its hottest around the border town of Vovchansk.

Russia has claimed that its forces have entered the town but Ukraine disputes