Pentagon chief in Kyiv amid questions over future US arms supplies

On a surprise trip to the Ukrainian capital, Secretary of Defense Lloyd Austin tried to rally support for Ukraine and even suggested Ukrainian forces would be "even more aggressive" in their fight against invading Russian forces throughout this winter.

Austin's buoyant prediction about the next phase of fighting comes after Ukrainian forces failed to achieve any decisive breakthrough during a six-month counteroffensive which came at a heavy cost for Ukrainian troops.

There is also serious concern in Ukraine about the longer-term future of U.S. military aid, with some Republicans wanting to halt support.

Speaking to reporters in Kyiv, Austin appealed to Congress to approve additional funding, calling it "a smart investment" in U.S. security.

"This is about ... not living in a world where a dictator can wake up one day and decide to annex the property of his peaceful neighbor," Austin told reporters in Kyiv.

However, Ukraine's ability to "take the fight to the enemy," as Secretary Austin put it, will depend, in large part, on the continuation of U.S. military aid, including ammunition.

The Biden administration is working with Congress to try and thrash out a broader funding deal that would include additional support for Ukraine.

However, weapons supplies are under additional pressure from the war in the Middle East.

Just ahead of Austin's trip, a Ukrainian official told ABC News that U.S. deliveries of NATO-standard artillery shells to Ukraine have fallen "by more than 30%" since Israel's war against Hamas in Gaza began last month.

155mm artillery shells are arguably the most important munition for Ukraine in its fight against invading Russian forces, and some U.S. stocks, which were designated for Ukrainian forces, have been diverted to Israel.

U.S. officials have claimed, on several occasions in recent weeks, that the supply of munitions to Israel would have no impact on the war in Ukraine.

"They (U.S. officials) were telling us it wouldn't influence the commitments (from the U.S.), but it did," a Ukrainian official said.

The official, who spoke to ABC News on condition of anonymity to discuss a sensitive matter, said supplies of these vital artillery shells make up "about 60-70% of Ukraine's overall supply."

A senior U.S. defense official, however, said the reduction in munitions has "absolutely nothing to do with what's happening in Gaza." Presidential drawdown authority packages "start to get put together weeks in advance, so there is no link between what's happening in Gaza to what's happening in Ukraine," the official said.

During his trip to Ukraine, Austin announced a new $100 million security aid package for Ukraine, which included an unspecified number of those NATO-standard 155mm artillery shells.

The latest U.S. military aid package is being drawn from existing funding which has already been approved by Congress. However, that pot of money is already running relatively low.

At the end of last month, Pentagon spokesman Pat Ryder said he was "confident" the United States could "continue to support both Ukraine and Israel."

Despite receiving a wide array of weaponry from Western partners, Russia has key battlefield advantages such as more artillery firepower, more explosive attack drones and superior types, and greater numbers of fighter jets and attack helicopters.

In recent weeks, Russian forces have been trying to steal the initiative on the battlefield by launching significant attacks on Ukrainian positions in eastern Ukraine.

Ukraine's top general, Valery Zaluzhny, in an interview with The Economist, recently described the war as being at "a stalemate."

Speaking in Kyiv on Monday, Secretary Austin insisted Ukrainian troops would "have the means ... to be successful in fighting in the wintertime."

However, the Ukrainian official who spoke anonymously to ABC News expressed real concern about U.S. weapons supplies in light of the disagreement in Congress.

"We're in big trouble ... basic munitions are not coming," the official said.

The official warned that Ukraine risked losing its position on the battlefield "at a very high price."