This is stated on the State Department's website.
Overall, Ukraine remains at a Level 4 danger rating, which advises American citizens against traveling to Ukraine altogether.
Specifically, the State Department recommends avoiding travel to frontline areas of Ukraine and regions along the border with Belarus due to the proximity of Russian and Belarusian troops, the closeness to active combat zones, and frequent shelling.
However, the danger level has been lowered to Level 3 for some regions of Ukraine, citing "air defense capabilities and distance from active combat zones" as the reasons.
The regions with a lower danger level include the western areas of Ukraine: Volyn, Lviv, Transcarpathian, Ivano-Frankivsk, Chernivtsi, Ternopil, Rivne, Khmelnytskyi, and Zhytomyr.
Nonetheless, the State Department emphasizes that the security situation can change rapidly, and travelers should be prepared to leave even safer regions immediately.