The comments on Friday from Pranay Vaddi, a senior director of the National Security Council, were the most explicit public warning yet that the United States was prepared to shift from simply modernizing its arsenal to expanding it.

Mr. Vaddi, speaking at the annual meeting of the Arms Control Association, a group that advocates limits on nuclear weapons, confirmed what officials have been saying in private conversations and closed congressional testimony for more than a year.

Fifteen years ago, President Barack Obama outlined a vision of moving toward a world without nuclear weapons, and he took steps to reduce their role in American strategy and defenses.

While the nation’s nuclear complexes were improved and made safer, and old weapons were swapped out for more reliable or updated versions, the United States insisted it was only “modernizing” its arsenal, not expanding it.

“Absent a change in the trajectory of the adversary arsenal, we may reach a point in the coming years where an increase from current deployed numbers is required, and we need to be fully prepared to execute if the president makes that decision,” he said.

The failure of Russia and China to engage in meaningful negotiations, Mr. Vaddi said, was “forcing the United States and our close allies and partners to prepare for a world where nuclear competition occurs without numerical constraints.”

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