President Biden’s top national security official made a secret trip to Kyiv on Wednesday, as Ukrainian soldiers holding off Russian troops are running out of munitions and U.S. aid remains stalled in congressional gridlock.

Jake Sullivan, the national security adviser, met with President Volodymyr Zelensky of Ukraine and his senior officials “to reaffirm the United States’ unwavering commitment to Ukraine in its self-defense against Russia’s brutal invasion,” said a national security spokeswoman, Adrienne Watson. “He stressed the urgent need for the U.S. House of Representatives to pass the national security supplemental to meet Ukraine’s critical battlefield needs.”

The covert trip showed the rising sense of urgency in the White House to pressure Congress to pass billions of dollars of aid for Ukraine, a financial package that the Biden administration says the country needs to defend itself against Russia.

The White House has tried, so far unsuccessfully, to push House Republicans to support a $60 billion emergency spending plan for weapons for Ukraine and to bolster armament production in the United States.

With that funding held back and future U.S. aid in limbo, the administration last week sent Ukraine a $300 million package that included air defense interceptors, artillery rounds, armor systems and an older version of the Army’s longer-range missile systems known as ATACMS. But that package is most likely going to hold off Russia for only a matter of weeks, U.S. officials have said.

“Ukrainian troops have fought bravely, are fighting bravely throughout this war,” Mr. Sullivan said when the package was announced, “but they are now forced to ration their ammunition under pressure on multiple fronts.”

Mr. Sullivan’s visit came one day after Defense Secretary Lloyd J. Austin III met with other backers of Ukraine in Germany to strategize on how to maintain military support for Kyiv.

“Ukraine’s battle remains one of the great causes of our time,” Mr. Austin said.