TEL AVIV — Even as Israel and Hamas agreed to suspend hostilities briefly on Thursday at the request of the United Nations, a senior Israeli military official said that his government was increasingly likely to order a ground invasion of the Gaza Strip that it had hoped to avoid.

Though Israel initially set limited goals of halting the rocket assaults against it and degrading Hamas, the Islamist movement that dominates Gaza, the group’s tenacity and surprisingly deep arsenal have led to widespread calls to expand the mission. The military official said only “boots on the ground” could eradicate terrorism from Gaza and indicated that Israel was even considering a long-term reoccupation of the coastal territory.

But with the Palestinian death toll reaching 214 on Wednesday, Israel and the Gaza militants agreed to end the violence for five hours on Thursday, from 10 a.m. to 3 p.m. For Israel, it was a move that might help mitigate international criticism of rising civilian casualties, and that carried little cost: The military warned that if Hamas or other groups “exploited” the “humanitarian window” to attack Israel, it would “respond firmly and decisively.”

Hours earlier, Israel called up 8,000 reservists in addition to the 42,000 troops already mobilized. With no progress reported from Cairo, where President Mahmoud Abbas of the Palestinian Authority had gone to discuss terms to end the fighting, Israel’s airstrikes intensified despite what the military official acknowledged were diminishing returns.

“Every day that passes makes the possibility more evident,” the military official said of a ground campaign. The official, who has been briefing Israeli ministers responsible for strategic decisions and spoke on the condition of anonymity under military protocol, said that his assessment was based on “the signals I get” and that the likelihood of an invasion was “very high.”