The United States is temporarily closing its embassy in Kyiv, the Ukrainian capital and moving embassy operations to Lviv, a city much farther from Russian territory, the State Department said on Monday, citing the Russian military buildup near Ukraine.

The department had said on Saturday that it would move most of its diplomatic staff in Kyiv to Lviv, but not all, indicating that it would keep the embassy operating. A department spokesman, Ned Price, declined to say at a news briefing on Monday how many people remained in Kyiv and were covered by the decision to shut the embassy.

Amid fears of a Russian invasion, the United States has strongly urged its citizens to leave Ukraine and has ordered some personnel and their families out of the country.

Kyiv lies within easy reach of Russian forces massed in western Russia and in Belarus. Lviv sits nearly 300 miles farther west, close to Ukraine’s border with Poland.

“I have no higher priority than the safety and security of Americans around the world, and that, of course, includes our colleagues serving at our posts overseas,” Secretary of State Antony J. Blinken said in a statement.

“With that in mind,” he added, “we are in the process of temporarily relocating our Embassy operations in Ukraine from our Embassy in Kyiv to Lviv due to the dramatic acceleration in the buildup of Russian forces. The Embassy will remain engaged with the Ukrainian government, coordinating diplomatic engagement in Ukraine.”

Mr. Blinken said: “The path for diplomacy remains available if Russia chooses to engage in good faith. We look forward to returning our staff to the Embassy as soon as conditions permit.”

Among those who have already relocated to Lviv, Mr. Price said, is Kristina A. Kvien, the embassy’s chargé d’affaires — the person in charge of an embassy when no ambassador is present. He said Ukrainian police would protect the compound in Kyiv.

“It is certainly our intention to return to that Embassy in Kyiv just as soon as it is safe for us to do so,” Mr. Price said.