The United States will soon be unable to supply Ukraine with certain types of ammunition essential to Kiev’s fight against the Russian invasion, as stocks are being depleted faster than they can be replaced.

Washington is by far the largest arms supplier to Ukraine since Russia began invading it on February 24, with more than $16.8 billion in military aid provided since that date.

But US stockpiles of some equipment “are reaching minimum levels needed for war plans and training” and could take years to replenish to pre-invasion levels, wrote Mark Cancian of the Center for Strategic and International Studies in a recent analysis.

Washington is “learning” from the conflict over ammunition needs in a great power war “far bigger” than expected, a US military official admitted on condition of anonymity.

American defense companies were forced to drastically reduce production in the 1990s when the United States cut defense spending after the collapse of the Soviet Union and their numbers dropped dramatically, from dozens to single digits.

Now the US government must persuade the industry to reopen production lines and resume production of items like Stinger anti-aircraft missiles that have not been manufactured since 2020.

Some US-supplied equipment has become a symbol of the war in Ukraine, such as the Javelin anti-tank weapons widely used by Kiev’s armed forces to repel Russian advances on the capital, and the Himars, a precision missile system now in use plays a key role in counter-offensives against Moscow’s troops in the east and south.

But US ammunition supplies for Himars – which fire GPS-guided missiles called GMLRS with a range of more than 80 kilometers (50 miles) – are dwindling.

“If the United States sends a third of that inventory to Ukraine (as it did with the Javelin and Stinger), Ukraine would receive 8,000 to 10,000 missiles. This inventory would probably last for several months, but when the inventory is exhausted, there are no alternatives,” said Cancian, who previously worked on weapons procurement for the US government.

“Production is around 5,000 a year. Although the United States is working to increase that amount, and money has recently been allocated to do so, it will take years,” he said, adding that older devices could help close the gap.

The United States has shipped about 8,500 Javelin missiles to Kyiv, but production of a weapon that has become a symbol of the Ukrainian resistance is only about 1,000 a year.

The US government ordered $350 million worth of the missiles in May, but again it will take several years to replenish stocks.

The United States has also shipped more than 800,000 NATO-standard 155mm artillery shells to Kyiv — three-quarters of the total shipped by all western countries, according to official Pentagon statistics.

The amount of shells Washington has provided “is probably close to the limit that the United States is willing to give without risk to its own combat capabilities,” Cancian said.

US production of these shells is currently 14,000 per month, but the Pentagon has announced that it intends to increase that number to 36,000 within three years. That would still only bring annual production to 432,000 – less than half of what was delivered to Ukraine in seven months.

And U.S. defense industry output is accelerating, Laura Cooper, a senior Defense Department official in charge of Russia and Ukraine, said Tuesday.

“The United States will continue to stand by the Ukrainian people and provide them with the security assistance they need to defend themselves for as long as is needed,” she said.