Mortar shells were fired from Gaza on Thursday just hours after a humanitarian cease-fire went into effect, the Israeli military said.

Initially, an Israeli police spokesperson said that two rockets had struck unpopulated areas in southern Israel, according to the Associated Press. No casualties were reported.

However, the Israeli army later confirmed that three mortars were fired from Gaza. The attack occurred just two hours into a planned five-hour cease-fire between Israel and Hamas, which agreed to a United Nations-backed temporary truce to allow residents of Gaza to gather supplies.

While it remains unclear how Israel will respond to the apparent mortar attack, a senior Israeli military official said on Wednesday that the government was strongly considering a ground invasion of the Gaza Strip, the New York Times reported. The Israeli government is also said to be mulling the possibility of a reoccupation of the region.

Israel had hoped that targeted military action would stop further rocket attacks and disrupt Hamas’ ability to wage a war. However, concerns over the organization’s doggedness and arsenal necessitate a “boots on the ground” response, the military official said.

“Every day that passes makes the possibility more evident,” he said, while adding that the likelihood of a ground invasion in the near future is “very high.” The Israeli military had previously stated that it would “respond firmly and decisively” if Hamas “exploited” the “humanitarian window.”

In the hours before the cease-fire, Israel mobilized an additional 8,000 reservists to supplement the 42,000 troops that were already active. The Israeli military official said that an invasion of Gaza would take “a matter of days or weeks,” but an occupation could last “many months.”

As the governments of Egypt and the U.S. continue to call for an end to hostilities, President Barack Obama reiterated American support for Israel.

“Israel has a fight to defend itself from rocket attacks, but over the past two weeks, we’ve all been heartbroken by the violence, especially the death and injury of so many innocent civilians in Gaza,” he said.

At least 214 Palestinians have been killed since military action began on July 7. One Israeli has also been killed.