"Hamas has security responsibility for Gaza... Rocket fire came from Gaza," State Department spokeswoman Marie Harf said, blaming the Palestinian Islamist group for renewed fighting.
Israel and Palestinian militants resumed on Tuesday despite ongoing talks to secure a lasting truce. Palestinian rockets hit southern Israel, and Israeli jets hit multiple targets inside Gaza.
"We are very concerned about today's development, condemn the renewed rocket fire and as we have said Israel has the right to defend itself against such attacks." Harf told reporters.
"We hope that the parties can reach an agreement on a sustainable ceasefire or, if necessary, agree to another extension of their temporary ceasefire so they can continue the conversation.
"But as of right now, with today's developments, we are very concerned and it is our understanding the ceasefire has broken down.