The Pentagon sent a warning to Congress saying that it is running low on funds to replace weapons that the U.S. has sent to Ukraine.

In a letter obtained by The Associated Press, the Pentagon said that it had already been forced to slow down when it comes to resupplying some troops. At the same time, the letter urged Congress to replenish funding for Ukraine. The letter came to light after Congress temporarily averted a government shutdown by passing a short-term funding bill over the weekend. However, the measure dropped all assistance for Ukraine in the battle against Russia.

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Pentagon Comptroller Michael McCord told House and Senate leaders that of the $25.9 billion Congress provided to replenish U.S. military stocks, there is only $1.6 billion left. The weapons that have been handed over to Ukraine from the U.S. as form of military air, include artillery, rockets and missiles, all of which was critical to Ukraine’s counteroffensive after Russia’s invasion into the country.

From its stockpiles, the U.S. has about $5.4 billion left to fund weapons and other military equipment.

The U.S. would have already run out of that funding much earlier had it not realized it had overvalued the equipment it had already sent to Ukraine. The realization ended up freeing up about $6.2 billion.

The Ukraine Security Assistance Initiative provides money to contract for future weapons. McCord said the U.S. has completely run out of long-term funding for Kyiv.

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“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 the U.S. will have to delay or curtail air defense weapons, ammunition, and other military equipment that are “critical and urgent now as Russia prepares to conduct a winter offensive” if more funding is not sanctioned.

Meanwhile, the White House spokesperson has issued a statement, “Putin believes he can outlast us, but he is mistaken. We will soon deliver another aid package to Ukraine.”