Secretary of State Antony J. Blinken met with President Volodymyr Zelensky of Ukraine in Kyiv on Tuesday, assuring him that American military aid is on its way at a “challenging time” for the country’s war effort against Russia.

The meeting was Mr. Blinken’s first since his morning arrival in Kyiv, and came under the shadow of Russian military gains in the country’s northeast. In an ornate conference room at his presidential offices on Bankova Street, Mr. Zelensky said that Mr. Blinken had come during “a tough period for the east of our country.”

The Ukrainian leader profusely thanked Mr. Blinken for the “crucial” $60.8 billion aid package for his country that President Biden signed last month after months of infighting among congressional Republicans. But Mr. Zelensky quickly added that Ukraine was still in need, pointing to the Russian military advances around the northeastern city of Kharkiv in recent days. Russian forces captured another village, Lukiantsi, overnight and bombed the city of Kharkiv on Tuesday morning, injuring four people.

Calling air defense a “deficit for us,” Mr. Zelensky said, “We really need it today, two Patriots for Kharkiv.” The Patriot is a U.S.-made surface-to-air missile system.

Mr. Blinken did not specifically respond to that request, but he told Mr. Zelensky that incoming American aid — some of which he said had already arrived — would “make a real difference on the battlefield.”

The unannounced visit was Mr. Blinken’s fourth to Ukraine’s capital since Russia’s full-scale invasion in February 2022. Mr. Blinken, who arrived on an overnight train from Poland, plans to deliver a speech later 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, according to a senior U.S. official.