Secretary of State Antony J. Blinken arrived in Kyiv on Tuesday morning for a visit meant to reaffirm American support for Ukraine but that might be shadowed by Russian military gains in the country’s northeast.

The unannounced trip, by overnight train from eastern Poland, was Mr. Blinken’s fourth to Kyiv since Russia’s February 2022 invasion of Ukraine. It comes about three weeks after President Biden signed a $60 billion aid package for Ukraine that Congress passed after months of infighting among House Republicans.

Mr. Blinken plans to deliver a speech in Kyiv on Tuesday celebrating the influx of American aid and portraying Russia’s failed effort to take control of the country as a strategic success for Ukraine, a senior U.S. official said.

Mr. Blinken will also underscore that Ukraine must continue to make progress on democratic governance and anti-corruption reforms if it wants to integrate with the West, the official said.

Mr. Biden and Mr. Blinken had warned for months that Congress’s delay in approving critically needed U.S. arms would leave Ukraine’s military vulnerable along an eastern battlefront that has been stalemated for months. The U.S. official declined to draw a direct connection between the delayed aid and Russia’s gains near the city of Kharkiv. But the official said it was clear that the gap in funding had left Ukraine, whose military is starved for ammunition and other critical equipment, weakened.

The official said that the Ukrainians had held their positions and were exacting a toll on the Russians, and that they were likely to make gains as U.S. assistance flows into the country.

Mr. Blinken plans to meet with Ukraine’s president, Volodymyr Zelensky, and other top officials. A second senior U.S. official would not say whether Russia had been notified in advance of Mr. Blinken’s visit. Russian forces have frequently attacked Kyiv with missiles and drones.

Mr. Blinken is the first senior Biden official to visit Ukraine since the passage of the congressional aid package. The White House national security adviser, Jake Sullivan, met with Mr. Zelensky in Kyiv in late March, before the aid passed.

Speaking at an event hosted by The Financial Times this month, Mr. Sullivan said that he expected Russia to make some short-term gains, but that the new U.S. aid would allow Ukraine to “hold the line” and eventually begin recapturing territory.