US Secretary of State Antony Blinken on Tuesday arrived in Kyiv on an unannounced visit meant to reassure Ukrainians of continued American support and flow of weapons as Russia pummels the northeastern Kharkiv region.

Marking his fourth visit to Kyiv since the start of the war in February 2022, Blinken arrived by overnight train from Poland.

Blinken will aim to reassure Ukrainian officials including President Volodymyr Zelenskiy of enduring American support and deliver a speech focused on Ukraine's future.

Kyiv has been on the back foot on the battlefield for months as Russian troops have slowly advanced, mainly in the Donetsk region to the south, taking advantage of Ukraine's shortages of troop manpower and artillery shells.

Russia's forces hold a significant advantage in manpower and munitions.

On Monday, US National Security Advisor Jake Sullivan said Washington was trying to accelerate "the tempo of the deliveries" of weapons to Ukraine to help it reverse its disadvantage. (Agencies)