A few hours after that strike, Israeli Prime Minister Ehud Olmert visited Ashkelon and said Israel would end the conflict if rocket fire on Israel stops and Hamas is unable to rearm.
"Anything else will be met with the Israeli people's iron fist," Olmert said. "We will continue as long as necessary to remove this threat from our heads."
Black smoke rose over Gaza City's suburbs, where the two sides skirmished throughout the night. At least 16 Palestinians were killed on Monday, Gaza health officials said. Among the dead was a militant killed in a northern Gaza battle.
The army announced Sunday that it was sending reserve units into Gaza to assist thousands of ground forces already in the territory. The use of reserves is a strong signal that Israel is planning to move the offensive, which Gaza officials say has killed some 910 Palestinians, into a new, more punishing phase.
Israel launched the offensive on Dec. 27, bombarding Gaza with dozens of airstrikes before sending in ground forces a week later. The operation is meant to halt years of Palestinian rocket attacks on southern Israel. Fighting has persisted despite international calls for a cease-fire. Thirteen Israelis, including 10 soldiers, have died.
Egypt, which often serves as a mediator between Israel and Hamas, has played a key role in trying to forge a cease-fire.
Much of the diplomacy focuses on an area of southern Gaza just across the Egyptian border known as the Philadelphi corridor that serves as a smuggling route, making Egypt critical to both sides in any deal. Israel wants those routes sealed as part of any peace deal.
Egypt's Foreign Ministry spokesman, Hossam Zaki, said his country's efforts to broker a truce were progressing and that the adversaries were jockeying for political gain ahead of a possible end to the fighting. He said, however, that Egypt could not provide certain guarantees that Israelis seek, such as a halt to rocket fire.
"We'll enhance our efforts, but this is not an issue between Israel and Egypt," Zaki told the BBC. "It is an issue between Israel and Gaza, and this is something that will have to be worked out, as the (U.N.) Security Council says, in Gaza."
International Mideast envoy Tony Blair was in Cairo on Monday, meeting with President Hosni Mubarak following talks with Israeli leaders on Sunday. Egypt has put forward a three-stage proposal to end the fighting.
"I think the elements of an agreement for the immediate cease-fire are there," Blair said, adding that, while more work needed to be done, he hoped to see a cease-fire "in the coming days."
Israel's representative to the talks, Defense Ministry official Amos Gilad, was in close contact with Egypt. But in a sign that more work is needed, he postponed a trip to Cairo, officials said.
With Israeli troops surrounding Gaza's main population centers, Israeli leaders have given mixed signals on how much further the army is ready to push, saying the operation is close to achieving its goals but vowing to press forward with overwhelming force.
"Israel is a country that reacts vigorously when its citizens are fired upon, which is a good thing," Foreign Minister Tzipi Livni told Israel Radio on Monday. "That is something that Hamas now understands and that is how we are going to react in the future, if they so much as dare fire one missile at Israel."
Israeli security officials say they have killed hundreds of Hamas fighters, including top commanders. However, there has been no way to confirm the claims, and Hamas officials say the group is determined to keep fighting.
The army also says Hamas has been avoiding pitched battles against the advancing Israelis, resorting instead to guerrilla tactics as its fighters melt into crowded residential areas.
In Monday's fighting, the army said it carried out more than 25 airstrikes, hitting squads of gunmen, mortar launchers and two vehicles carrying Hamas militants.
It also said ground troops came under fire from militants holed up inside a mosque. An Israeli aircraft attacked the squad, and Israeli troops then took over the mosque, confiscating rockets and mortar shells.
Israeli leaders are expected to decide in the next day or two on whether to push the offensive into a third phase — in which the army takes over larger areas of Gaza.
A push into densely crowded urban areas would threaten the lives of many more civilians. More than 20,000 Palestinians have already fled Gaza's rural border areas and crowded into nearby towns, staying with relatives and at U.N. schools turned into makeshift shelters.
International aid groups have repeatedly said Israel must do more to protect Palestinian civilians, who are believed to make up about half of the dead.
Human Rights Watch has accused Israel of firing artillery shells packed with the incendiary agent white phosphorus over populated areas of Gaza. The chemical, used for creating smoke screens and for illuminating battlefields at night, ignites when it comes in contact with oxygen and can cause serious burns and spark fires as it drifts to the ground in long trails of smoke.
Marc Garlasco, a military analyst working for the rights organization, said he witnessed such shelling from the Gaza-Israel border last weekend. He reviewed AP Television News footage on Monday of similar midair fire that he said was white phosphorus.
"You basically have what looks like the head of a jellyfish and the tentacles coming down on fire," he said. "It'll burn for approximately five to 10 minutes, depending on atmospheric conditions and this causes extreme fire and the potential for civilian harm."
The Israeli army refuses to say whether it's using phosphorus, saying only it is "using its munitions in accordance with international law."
White phosphorus is not illegal. Under customary laws of war, however, Israel would be expected to take all feasible precautions to minimize the impact on civilians, Human Rights Watch said.
In addition to Egypt, European envoys have been pressing efforts to negotiate an end to the war even though Israel and Hamas have ignored a U.N. Security Council resolution calling for an immediate and durable cease-fire.
Israel is demanding an end to years of rocket attacks, as well as international guarantees to prevent Hamas from smuggling weapons into Gaza through the porous Egyptian border. Israel has been bombing tunnels that run under that border.
In an e-mail message Monday, Hamas leader Ismail Radwan said his group would not consider a cease-fire before Israel stops its attacks and pulls back from Gaza. He also demanded opening of all border crossings, emphasizing the Rafah crossing with Egypt.
In Paris, the French foreign minister, Bernard Kouchner, said Monday that European military observers should be sent to Gaza to monitor any eventual cease-fire. He said that "neither the Egyptians nor the Israelis want international observers on their territory for the moment."
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Barzak reported from Gaza City and Federman from Jerusalem.
