Ukraine has sent reinforcements to Bakhmut, a senior Ukrainian official said on Wednesday, signaling the intensity of fighting in a city that has become a crucible in the east of the country as Russian forces gradually tighten their grip.

Speaking on Ukrainian television, the official, Deputy Defense Minister Hanna Maliar, did not say how many troops were being sent or for what purpose, leaving open multiple possibilities. The reinforcements could be needed in case of a Ukrainian withdrawal, or they could be part of an effort to continue to defend Bakhmut. Kyiv could be seeking to keep Russian forces tied up there so they cannot redeploy to other battles.

Ukrainian soldiers have for months held out in Bakhmut, and Kyiv has used their resistance as a symbol of the country’s broader defiance a year after Russia launched its full-scale invasion. But in recent weeks, Russian forces, including large numbers of newly mobilized recruits, have been rushed to the front line in the east, helping Moscow seize villages and towns around Bakhmut and surround the city on three sides.

President Volodymyr Zelensky has used the slogan “Hold Bakhmut,” a mantra that has become popular in Ukraine. But amid reports that fighting has broken out in parts of the city itself, the language he has used to describe the battle has shifted.

“The most difficult situation is still Bakhmut and the battles that are important for the defense of the city,” Mr. Zelensky said on Tuesday in his nightly address, the second day in a row that he has referred to problems facing the city’s defenders. “The intensity of fighting is only increasing.”

On Monday, he said that Russian forces were destroying anything that could shelter the forces fighting to defend the city.

Much of the fighting in and around Bakhmut has been conducted by troops from the Wagner Group, a mercenary force whose leader, Yevgeny V. Prigozhin, has close ties to President Vladimir V. Putin of Russia.

Mr. Prigozhin said on Wednesday in a broadcast audio message that there was no sign that Ukrainian forces were withdrawing from the city. Moscow turned its attention to the capture of Bakhmut in the summer after it had seized two cities in the nearby Luhansk region, but it now stands as the most protracted battle of the full-scale war and both sides have sustained heavy casualties.