"You have a right to go in and take out those tunnels," Kerry told Fox News. "We completely support that. And we support Israel's right to defend itself against rockets that are continuing to come in."
Israel, which has accused Palestinian militant group Hamas of using civilians as human shields by launching rockets from residential areas, sent ground forces into the Gaza Strip on Thursday after 10 days of air, naval and artillery barrages failed to stop the salvoes.
At least 62 Palestinians were killed on Sunday in Israeli shelling of one Gaza neighborhood.
Kerry called on Hamas to consider a ceasefire. "It is important for Hamas to now step up and be reasonable and understand that (if) you accept the ceasefire, you save lives," he said.
In a separate interview on CNN, Kerry said President Barack Obama will ask him to go to the Middle East soon to aid in efforts to secure a ceasefire.