"The Department will not execute these transfers until 15 days have passed following submission of this notification," McCloud said in the letter dated December 15.
"Once these funds are obligated, the Department will have exhausted the funding available to us for security assistance to Ukraine."
McCloud in the letter to Congress urged lawmakers to approve Biden's $106 billion supplemental package to address US national security concerns, which includes some $60 billion for Ukraine.
A Pentagon spokesperson told Sputnik that $4.4 billion of "restored'' presidential drawdown authority currently remains available for Ukraine - funding that remains as a sort of credit after the Pentagon realized it mistakenly did not send Ukraine the appropriate amount of military equipment that amounted to the value in several previous packages.
The United States will only be able to provide Ukraine with one more additional aid package before the end of the year absent approval for new funding, according to McCloud.
The letter comes as US President Joe Biden announced he was releasing a $200 million package to Ukraine amid the ongoing congressional blockade imposed by Republican lawmakers. Although insider reports suggest lawmakers are working toward a potential deal before year's end, it remains uncertain whether officials will be able to hammer out an agreement by deadline.