The Biden administration announced another package of military aid to Ukraine worth up to $300 million on Tuesday after months of warning there was no money left, with officials saying the new funding became available as a results of savings made in weapons contracts.

National security adviser Jake Sullivan announced the package in a briefing at the White House on Tuesday afternoon.

“When Russian troops advance, and its guns fire, Ukraine does not have enough ammunition to fire back. That’s costing terrain. It’s costing lives. And it’s costing us, the United States and the NATO alliance, strategically,” Sullivan said.

President Joe Biden later expressed a similar sentiment, saying the package is “not nearly enough,” and Congress needs to pass additional funding.

“We must act before it literally is too late, before it’s too late, because as Poland remembers, Russia won’t stop at Ukraine,” Biden said, speaking alongside the Polish prime minister and president. “Putin will keep going, putting Europe, the United States the entire free world at risk in my view.”

Pentagon spokesman Maj. Gen. Pat Ryder said Tuesday the package would include “Stinger anti-aircraft missiles, additional ammunition for HIMARS, 155 millimeter artillery rounds, including high explosive and dual purpose improved cluster munition rounds, 105 millimeter artillery rounds, AT4 anti-armor systems, additional rounds of small arms ammunition, demolitions, munitions for obstacle clearing, spare parts, maintenance and other ancillary equipment.”

In explaining how the Defense Department now has money available for Ukraine aid, a senior defense official said, “We had savings come in that will allow us to offset the cost of a new drawdown package.”

The Pentagon has had approximately $4 billion in drawdown authority left to send to Ukraine – weapons and equipment pulled directly from Defense Department stocks. But the Pentagon was reluctant to use that funding, because there was no replenishment money left to refill the US inventories.

The newfound savings – the result of “good negotiations” and “bundling funding across different things,” according to a second senior defense official – provided the Pentagon with an additional $300 million to use as replenishment funding, to backfill the aid sent to Kyiv.

Sullivan said the new package was possible “because of unanticipated cost savings in contracts that DOD negotiated to replace equipment we’ve already sent to Ukraine through previous drawdowns.”

The second official gave one example of being able to buy 25mm ammunition at a cheaper cost than originally expected after contract negotiations with the vendor.

But the official made clear this is not a sustainable long-term solution to providing much-needed weaponry to Ukraine as Republican leaders in the House continue to refuse to bring a bill that would provide additional military aid up for a vote.

“This is a bit of an ad-hoc or one time shot,” said the first official. “We don’t know if or when future savings will come in. And we certainly can’t count on this as a way of doing business.”

“We weren’t broke at the time, but now we are.”

Sullivan said the package would only provide Ukraine enough ammunition to last weeks, and perhaps only “a couple of weeks” and will “not prevent Ukraine from running out of ammunition in the weeks to come.”

“It goes without saying, this package does not displace and should not delay the critical need to pass the bipartisan national security bill,” Sullivan said.

The last Ukraine aid package from the US was announced in late December. At the time, the Pentagon said in a letter to Congress that the Defense Department “will have exhausted the funding available to us for security assistance” after the package was announced.

Asked if this new aid package will reduce pressure on Congress to pass a supplemental bill that includes billions in aid for Ukraine, the second official said, “It shouldn’t.”

“The supplemental is absolutely vital for our readiness, as well as Ukraine winning this conflict. This doesn’t change that at all. It is a relatively small package to give Ukraine, the minimum of what it needs for a short amount of time.”

This is not the first time the Pentagon has announced additional, unexpected sources of funding for Ukraine. Last year, the Defense Department announced that it had discovered an accounting error that led to DoD overvaluing the amount of aid it was providing to Ukraine by $6.2 billion. That extra money provided a cushion to the department that allowed it to draw out military assistance to Ukraine for longer than anticipated, CNN previously reported.

Without the support and weapons supplies from the US, Ukraine has lost ground in the war with Russia, outnumber and outgunned by an adversary that has fully shifted its economy to a war-time footing. Last month, Russian forces raised their flag in Avdiivka in eastern Ukraine after a months-long assault.

As Ukraine lost ground, the Biden administration urged the House of Representatives to act on the $60 billion supplemental, which has already passed the Senate.

“We believe that the support is still there if the House is allowed to vote,” said the second defense official. But the House’s Republican leadership has refused to bring the bill for a vote, forcing the Pentagon to think differently about how to send aid to Ukraine.

“You’ve got to take a different risk calculus about going any further if it’s no longer assured that you’re going to get help,” the official said.