WASHINGTON (AP) — The Pentagon is warning Congress that it is running low on money to replace weapons the U.S. has sent to Ukraine and has already been forced to slow down resupplying some troops, according to a letter sent to congressional leaders.

The letter, obtained by The Associated Press, urges Congress to replenish funding for Ukraine. Congress averted a government shutdown by passing a short-term funding bill over the weekend, but the measure dropped all assistance for Ukraine in the battle against Russia.

Pentagon Comptroller Michael McCord told House and Senate leaders there is $1.6 billion left of the $25.9 billion Congress provided to replenish U.S. military stocks that have been flowing to Ukraine. The weapons include millions of rounds of artillery, rockets and missiles critical to Ukraine’s counteroffensive aimed at taking back territory gained by Russia in the war.

In addition, the U.S. has about $5.4 billion left to provide weapons and equipment from its stockpiles. The U.S. would have already run out of that funding if the Pentagon hadn’t realized earlier this year that it had overvalued the equipment it had already sent, freeing up about $6.2 billion. Some of that has been sent in recent months.

McCord said the U.S. has completely run out of long-term funding for Kyiv through the Ukraine Security Assistance Initiative, which providesmoney to contract for future weapons.

“We have already been forced to slow down the replenishment of our own forces to hedge against an uncertain funding future,” McCord said in the letter. “Failure to replenish our military services on a timely basis could harm our military’s readiness.”

He added that without additional funding now, the U.S. will have to delay or curtail air defense weapons, ammunition, drones and demolition and breaching equipment that are “critical and urgent now as Russia prepares to conduct a winter offensive.”

President Joe Biden said Sunday that while the aid will keep flowing for now, time is running out.

“We cannot under any circumstances allow America’s support for Ukraine to be interrupted,” Biden said. “We have time, not much time, and there’s an overwhelming sense of urgency.”

Mark Cancian, a senior adviser at the Center for Strategic and International Studies, said if the aid doesn’t keep flowing, Ukrainian resistance will begin to weaken.

“If there’s no new money, they’re going to start feeling it by Thanksgiving,” he said.

The short-term funding bill passed by Congress lasts only until mid-November. And McCord said it would be too risky for the Defense Department to divert money from that temporary funding bill to pay for more aid to Ukraine.

Many lawmakers acknowledge that winning approval for Ukraine assistance in Congress is growing more difficult as the war grinds on and resistance to the aid from the Republican hard-right flank gains momentum.