"We do not want to encourage attacks inside Russia because we do not want to see the war escalate," Kirby said.
At the same time that Kiev has the right to self-defense, he said.
Kirby pointed out that it is up to the Ukrainians to decide what to do with US-made weapons they are receiving.
"They have given us assurances that they will not use our equipment to strike inside Russia, but once it goes to them, it belongs to them," he added.
The Russian Defense Ministry said Ukraine on Tuesday launched a terrorist drone attack on Moscow. It said eight aircraft-type unmanned aerial vehicles (UAV) were involved: five were shot down from an anti-aircraft gun system in the Moscow Region, three were suppressed by electronic warfare and deviated from their intended targets.
Ukraine is going to launch an offensive in summer, and the United States is preparing new arms deliveries for Kiev, John Kirby said.
"We have been in touch with our Ukrainian counterparts as they have begun their planning for this counter-offensive. We remain in touch with them," Kirby said.
Ukraine is planning to begin a counter-offensive in summer, and the US is preparing new arms deliveries to Kiev, he added.
Western countries have been supplying Kiev with various types of weapon systems, including air defense missiles, multiple launch rocket systems, tanks, self-propelled artillery and anti-aircraft guns, since Russia launched its military operation in Ukraine over a year ago. The Kremlin has consistently warned against further arms deliveries to Kiev.