The move marks a significant escalation in violence between Palestinian militants and Israeli forces, which had been worsening in recent days.
The military said in a statement early on Friday, without providing further details, that “air and ground” troops had “begun attacking in the Gaza strip”.
It later issued a statement clarifying that there were no troops inside the Gaza Strip, suggesting it was not a ground invasion but artillery and tank fire from the border.
Dramatic images showed masses of red flames illuminating the skies, as the deafening blasts from the outskirts of Gaza City jolted people awake.
Prime Minister Benjamin Netanyahu said in statement: “I said we would extract a very heavy price from Hamas.
“We are doing that, and we will continue to do that with heavy force.”
The fighting broke out late on Monday when Hamas, claiming to be the defender of Jerusalem, fired a barrage of long-range rockets toward the city in response to what it said were Israeli provocations.
Israel quickly responded with a series of airstrikes and, since then, has attacked hundreds of targets in Gaza.
The strikes set off scores of earth-shaking explosions across the densely populated territory. Gaza militants have fired nearly 2,000 rockets into Israel, bringing life in the southern part of the country to a standstill.
Several barrages targeted the seaside metropolis of Tel Aviv, some 45 miles away.
Gaza’s Health Ministry said the death toll has climbed to 109 Palestinians, including 28 children and 15 women, with 621 people wounded.
The Hamas and Islamic Jihad militant groups have confirmed 20 deaths in their ranks, though Israel says that number is much higher.
Seven people have been killed in Israel, including a six-year-old boy.
The current eruption of violence began a month ago in Jerusalem, where heavy-handed Israeli police tactics during Ramadan and the threatened eviction of dozens of Palestinian families by Jewish settlers ignited protests and clashes with police.
A focal point of clashes was Jerusalem’s Al-Aqsa Mosque, built on a hilltop compound that is revered by Jews and Muslims.
Israel regards Jerusalem in its entirety as its capital, while the Palestinians want east Jerusalem to be the capital of their future state.
The violent clashes between Arabs and Jews in Israel resulted in scenes not witnessed in more than two decades.