"Battlefield commanders are making some really tough decisions right now," Kirby told reporters. "We know that in some units they are marshalling their ammunition very, very closely because they're beginning to get worried about running out."
Kirby added that Ukrainian forces are also assessing the availability of their longer range systems that they are using on the battlefield.
In December, the Biden administration ran out of funding to provide security assistance to Ukraine and Congress has yet to approve more funding.
Congress has been involved in a weeks-long debacle over the bipartisan deal reached on a national security supplemental package that includes some $60 billion for Ukraine.
The measure received no support from Republican leadership in the Senate and House of Representatives, which creates further uncertainty about when the United States will be able to provide additional aid to Ukraine.