The State Department conceded for the first time on Thursday that it delayed making a $400 million payment to Iran for several hours in January “to retain maximum leverage” and ensure that three American prisoners were released the same day.

For months the Obama administration had maintained that the payment was part of a settlement over an old dispute and did not amount to a “ransom” for the release of the Americans. Instead, administration officials said, it was the first installment of the $1.7 billion that the United States intends to pay Iran to reimburse it for military equipment it bought before the Iranian revolution that the United States never delivered.

But at a briefing on Thursday, John Kirby, the State Department spokesman, said the United States “took advantage of the leverage” it felt it had that weekend in mid-January to obtain the release of the hostages and “to make sure they got out safely and efficiently.”

Republicans opposed to the nuclear deal President Obama reached with Iran have described the payment as ransom and a further sign of his administration’s feckless dealings with Tehran.