US Secretary of State Antony Blinken arrived in Kyiv on Tuesday morning. He wrote that he had come to show support.
The visit was first reported by Reuters, but there was no official information about the visit. The agency wrote that the US official arrived in Kyiv by train.
"Today, I returned to Kyiv to demonstrate our unwavering support for Ukraine as it defends its freedom from Russian aggression," the secretary of state wrote in a tweet.
According to officials close to the top US diplomat, the purpose of the visit is to "send a strong reassuring message to Ukrainians who are going through a difficult time".
"The secretary's mission is to explain how the aid will be used to strengthen defences and how it will then allow us to regain the initiative on the battlefield," a State Department source said on condition of anonymity.
Blinken travelled from Poland by train for nine hours, the BBC reports. On Monday, he flew from the United States to the Polish city of Rzeszów, whose airport is now a major logistics hub for defence supplies and humanitarian aid to Ukraine.
Asked by the BBC whether the delay in arms deliveries by Congress could have contributed to Moscow's decision to launch the offensive in Kharkiv Region, the anonymous official said they would not "draw any direct correlation" but added that "the Ukrainians are certainly in a more difficult position because they have been waiting for this assistance for a long time."
Washington hopes that the increase in weapons will allow Ukraine to move to "active defence" against the Russians.