The Defense Department fund used for buying Ukraine new weapons and replacing U.S. arms sent to that country is about to be emptied, the Pentagon comptroller told Congress’ defense committees in a Friday letter.

The missive from Defense Department comptroller Mike McCord to the Armed Services committees and Defense Appropriations subcommittees highlights the perilous stakes for war-ravaged Ukraine of a congressional impasse over war funding. Congress has been unable to appropriate money to help Ukraine or Israel due to a dispute over U.S. immigration policies.

The Pentagon has $1.1 billion left in appropriations in the fund for buying new weapons, which is known as the Ukraine Security Assistance Initiative. But officials plan to spend that money within 15 days to pay back the Defense Department to restore goods and services previously sent to Ukraine. These could include everything from weapons to training to logistics support.

“Once these funds are obligated, the Department will have exhausted the funding available to us for security assistance to Ukraine,” said McCord in the letter, obtained by CQ Roll Call and first reported Monday by Bloomberg News.

The Pentagon has a separate source of funding for Ukraine in the form of so-called Presidential Drawdown Authority, which authorizes the department to send its own weapons and services up to a certain value. At last count, the department had used up all but $4.6 billion of that authority.

The Pentagon is hesitant to send billions of dollars of its weapons and services abroad without knowing that appropriations are in place to backfill inventories.

The department plans to announce only one additional drawdown after this week “in order to protect U.S. military readiness,” McCord said.

“It is essential that Congress act without delay on the Administration’s pending supplemental request,” he wrote. “Doing so is in our clear national interest, and our assistance is vitally needed so Ukraine can continue its fight for freedom and to ensure Russia continues to fail in Ukraine.”

National Security Council spokesman John Kirby cited McCord’s letter in a press briefing Monday.

Kirby said the money spent to replenish U.S. stocks or buy Ukraine new weapons goes to American defense contractors.

He also noted that Russia launched the latest in its “nightly barrage” of drone and cruise missile attacks in the last 24 hours. Moreover, he said, Russian troops are mobilizing for a ground offensive in eastern Ukraine.

“Ukraine still needs our help,” Kirby said. “And it’s well past time for Congress to act to stand up for freedom and democracy in defense of our own national security interests, which are very much at play.”

Niels Lesniewski contributed to this report.