Secretary of State Antony J. Blinken arrived in Kyiv, the capital of Ukraine, on Wednesday morning at a precarious moment in the nation’s defense against the Russian invasion.

The visit comes as the Biden administration struggles to curtail aid to Russia from Iran, North Korea and China, and as the U.S. presidential elections loom over the conflict.

Mr. Blinken stepped off a private overnight train from Poland that also carried his British counterpart, David Lammy. The two men are scheduled to meet throughout the day with Ukrainian officials, including the country’s new top diplomat, Andrii Sybiha, who was appointed last Thursday by President Volodymyr Zelensky of Ukraine as part of a major overhaul of the government’s top ranks.

The joint visit by Mr. Blinken and Mr. Lammy was aimed at presenting a strong show of solidarity for Ukraine — a message to both Mr. Zelensky and President Vladimir V. Putin of Russia, as well as the rest of the world.

While both men have made comments in recent months theoretically indicating an openness to negotiating a settlement to the over 30-month full-scale war begun by Moscow, Mr. Putin’s vision of a reconstituted Russian empire is untenable to Ukraine, and U.S. officials say he has shown no signs of wanting to reach a peace agreement. Mr. Putin’s grand invasion of Ukraine in 2022 was a continuation of his annexation of Crimea and military offensive in eastern Ukraine that began in 2014.

Mr. Zelensky said last month that his multiphase plan for peace involves forcing Russia to end the war on terms that are “fair” to Ukraine.