WASHINGTON (AP) — The U.S. has learned a great deal about Russian military shortfalls and capabilities in the first two months of the war in Ukraine, top Pentagon leaders told Congress Tuesday. But they warned that Moscow is learning from its mistakes as the war shifts to a new phase, and that will shape the artillery and other weapons systems the U.S. will provide.

Defense Secretary Lloyd Austin and Gen. Mark Milley, chairman of the Joint Chiefs of Staff, told a Senate Appropriations subcommittee that if Congress approves funding, the most critical things that Ukraine needs are anti-tank, anti-aircraft and shoulder-fired surface-to-air missiles. Milley added that with the fighting now concentrated in the eastern Donbas region, Ukrainian forces also need more tanks and other mechanized vehicles, which the U.S. and other nations are providing.

The coming weeks, they said, will be crucial.

Prior to Russia’s invasion of Ukraine in late February, the U.S. had provided about $1 billion in weapons and gear to the Ukrainian military, and had been training troops for years. Since the invasion, the U.S. has committed another $3.7 billion in weapons and other aid, and is seeking a $33 billion supplemental appropriation from Congress that includes a wide range of military and other support.

Senators, including Sen. Patty Murray, D-Wash., pressed Austin and Milley on whether weapons are actually getting out to the troops on the front lines, or if they are being diverted or hoarded. Austin said it’s difficult to know since there are no U.S. personnel on the ground in Ukraine to monitor the weapons flow. But he said they talk to their counterparts in Ukraine regularly, and stress the need for accountability in weapons distribution.

Austin pointed to early failures by Russia, including almost immediate struggles with logistics, and difficulties getting food, water and supplies to troops.

“As we saw things unfold on the ground, we saw them not able to support themselves logistically, we saw them make some bad assumptions at the beginning of this, we saw them fail to integrate aerial fires with their ground maneuver, and just a number of missteps,” Austin said. “ I attribute a lot of that to lack of leadership at the lower level.”

The leadership problems, he said, forced Russia to send higher ranking generals to the battlefront, where “many” have been killed.

Austin said the U.S. expects to see some of the same mistakes as the fighting in the Donbas and across southern Ukraine escalates, as Russia tries to wrest control of a solid stretch of land from the east, through Mariupol, along the Sea of Azov to Crimea, the Black Sea peninsula seized by Russian and annexed in March 2014.

But, he said, “they will learn from what they did in the early stages of this fight. And we’ll see them improve their logistical efforts. And we’ll see them improve their massing of fires and that sort of business. But some things they won’t be able to correct.”

Austin and Milley said that Russia’s failure to train young commanders to make decisions, has led to a very top-heavy organization that hasn’t been as nimble and effective as the Ukrainian forces. Milley said the U.S. and other Western nations taught Ukrainian forces about mission command and decentralized control and tactics, which are more successful on a dynamic battlefield.

He also said the U.S. “opened up the pipes” and sent a “significant amount of intelligence” to Ukraine both before the invasion, and as the fight has gone on.

In a related matter, President Joe Biden has nominated Army Gen. Christopher G. Cavoli to be the next top general for Europe, and to serve as the Supreme Allied Commander for NATO. Cavoli is currently serving as commander of U.S. Army Europe-Africa. His nomination now goes to the U.S. Senate for confirmation. He would replace Air Force Gen. Tod D. Wolters.