President Trump said on Monday that the United States would send more weapons to Ukraine to help it fend off Russia’s invasion, arguing that Moscow’s recent assault on Ukrainian cities left him with little choice.

Mr. Trump’s comments appeared to signal a reversal from the president after his administration paused some arms transfers to the country just last week, raising fears that the United States was retrenching its support. Instead, Mr. Trump said on Monday that he had grown unhappy with President Vladimir V. Putin of Russia, who has frustrated Mr. Trump’s hopes to broker a cease-fire.

“We’re going to send some more weapons,” Mr. Trump told reporters at the White House. “We have to. They have to be able to defend themselves. They’re getting hit very hard. Now they’re getting hit very hard. We’re going to have to send more weapons, defensive weapons, primarily, but they’re getting hit very, very hard. So many people are dying in that mess.”

Those statements were a remarkable turnaround for Mr. Trump, who has often expressed skepticism of U.S. aid to Ukraine and just months ago dressed down President Volodymyr Zelensky of Ukraine in the Oval Office, claiming he had been insufficiently grateful for America’s support.

The White House acknowledged last week that the administration had paused the delivery of some air defense interceptors and precision-guided bombs and missiles to Ukraine, citing Pentagon concerns that U.S. weapons stockpiles were dwindling. The decision was described at the time by a White House spokeswoman as an assessment of munitions provided around the globe.

Two people briefed on the pause, who were not authorized to discuss the matter publicly, said that Mr. Trump had directed the Pentagon to review available munitions stockpiles around the time the United States conducted surgical bombing attacks on three Iranian nuclear weapons sites. From there, someone at the Pentagon — classifying the munitions in different categories — halted at least some of what was scheduled to be sent to Ukraine, one of the people briefed on the matter said.