Medics said an Israeli air strike on a tent encampment housing displaced families killed at least four people, while health officials said another strike killed five in Khan Younis in the south.
"In recent hours, the IDF has begun striking in response to Hamas's blatant violation of the ceasefire agreement yesterday in the Beit Hanoun area," an Israeli military official said, adding that "terrorists emerged from a tunnel east of the yellow line".
The official called Sunday's strikes "precise" and in line with international law.
The Palestinian militant group had committed more than six violations of an October ceasefire, the official said, including deploying east of the "Yellow Line" agreed under the ceasefire to demarcate Israeli- and Hamas-controlled areas.
"Crossing the yellow line in the vicinity of IDF troops, while armed, is an explicit ceasefire violation - and demonstrates how Hamas systematically violates the ceasefire agreement with intent to harm IDF troops," the official said.
Israel and Hamas have repeatedly accused each other of violating the ceasefire deal, a key element of US President Donald Trump's plan to end the Gaza war.
On Saturday, the military said it had identified armed "terrorists" near IDF personnel operating in the northern Gaza Strip.
The IDF said it continued to destroy underground tunnels in the northern Gaza Strip in accordance with the agreement.
It said it observed several gunmen emerging from what it said was a tunnel and entering beneath the rubble of a building east of the Yellow Line.
The military said air force aircraft had attacked the building and eliminated two gunmen and that it was likely that additional militants were eliminated in the strike.
The Gaza health ministry said at least 600 Palestinians have been killed by Israeli fire since the Gaza deal began.
Israel said four soldiers were killed by militants in Gaza in the same period.
