Israel has mobilized 20,000 reservists for a possible ground operation into Gaza, but for the time remains focused on maximizing its air campaign, Lerner said. A ground invasion could lead to heavier civilian casualties on the Palestinian side and put Israeli ground forces in danger.
Neither side is showing any sign of halting their heaviest fighting since an eight-day battle in late 2012. Israel says that Hamas must cease rocket fire from Gaza for Israel to consider a truce. Militants have fired hundreds of rockets, striking across the length of Israel and disrupting life across the country.
No Israelis have been seriously harmed as the "Iron Dome" defense system has intercepted at least 70 of the projectiles destined for major cities. The system is designed to intercept rockets headed toward populated areas, while allowing others to fall in open areas.
"The ground option needs to be the last option and only if it is absolutely necessary. It is a carefully designed plan of action," Lerner said.
The Israeli security Cabinet met to discuss its next moves.
Secretary of State John Kerry, in Beijing for a summit with Chinese leaders, said the U.S. is trying to stem the surging violence in a way that allows the Jewish state to continue defending itself from Hamas rocket fire. He called it a "dangerous moment" for the Mideast.
Kerry said he has spoken to Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas.
Israeli Defense Minister Moshe Yaalon said the operation was going according to plan, with Israel targeting various Hamas interests.
"The military's successes so far have been very significant," he said. "We will continue until they understand that this escalation is not beneficial to them and that we will not tolerate rocket fire toward our towns and citizens."
Palestinian medical officials said one strike early Thursday struck a home in the southern Gaza city of Khan Younis, killing eight members of the Al Haj family. Tractors cleared away large piles of debris from the demolished building as one man laid atop a mattress and blankets that remained.
An earlier strike killed eight people who were at a Gaza beach cafe watching the World Cup semifinal match between Argentina and the Netherlands, Mahmoud Sawali said, adding at least two of his brothers were among the dead.
"We only ask of help from God. Here I have two brothers who are martyrs, and I'm looking for the third," he said.
The Israeli military said it was investigating both incidents. It also said it struck a car in Gaza carrying three Islamic Jihad militants involved in firing rockets. The militant group confirmed that its men were killed in the strike. Hamas officials said that the Palestinian side of its border crossing with Israel had also been destroyed in Israeli airstrikes.
The Health Ministry in Gaza has reported 85 deaths, including 22 on Thursday. It says about half were women and children though the exact breakdown could not immediately be confirmed.
Israel accuses militants of deliberately endangering civilians by using homes and other civilian buildings for cover. The military has also directly targeted the offices and homes of known militants that it says are used as command centers. The military says it contacts the families first to ask civilians to evacuate before striking its targets. Many of the dead appear to be relatives of wanted militants according to names of the dead provided by the Palestinians.
Hamas is firing rockets from "within houses and streets and neighborhoods which are populated with civilians ... exposing these civilians to retaliation and to backfire," Israeli Foreign Ministry spokesman Yigal Palmor said.