Tanks, armoured vehicles and troops have been massing along the boundary for several days.
With the Israeli offensive on Gaza in its seventh day, the death toll now stands at more than 428 dead and 2,100 injured. Among Friday's dead were three girls who had been playing outside.
In response Hamas fighters launched more than 20 rockets into southern Israel on Thursday and seven on Friday, some landing as far as the port city of Ashdod, more than 30km from Gaza. An Israeli woman was injured in Friday's rocket fire, the Israeli army said.
Jacky Rowland, Al Jazeera's correspondent reporting from southern Israel, said: "This is two days of very heavy bombing, particularly in the north now, which has made people wonder whether this could be a foretaste of a ground offensive.
"It could be that Israeli forces are trying to clear any obstacles, notably landmines that could potentially lie i the path of advancing tanks if and when the ground offensive begins.
"But the real question now is not if, but when. Many people in Israel are wondering 'what are we waiting for'."
Hamas leader killed
In the strikes on Gaza on Thursday, Israeli jets pounded the border town of Rafah in the south of the territory and the Jabaliya refugee camp to the north.
In raids on Jabaliya, a senior Hamas official was among more than a dozen people killed when a single one-tonne bomb dropped from an Israeli jet destroyed his home.
Nizar Rayyan is the most senior Hamas official killed since Israel unleashed its massive bombardment on Gaza seven days ago.
Palestinian medics said 13 members of Rayyan's family, including his four wives and 10 children, were killed in the attack.
Hamas officials hit back at Israel after the attack saying the assault on Gaza would fail.
"The blood of Sheikh Nizar Rayyan and the blood of other martyrs will never be wasted and the enemy will pay a heavy price for the crimes it has committed," Ayman Taha, a Hamas official, said.
Rayyan, 51, had refused to take security precautions despite Hamas figures being at risk of assassination. He held a PhD in Islamic studies and lectured at the Islamic University in the Gaza Strip.
Ayman Mohyeldin, Al Jazeera's correspondent in Gaza, said the killing of Rayyan comes at a time when international organisations are saying Israel's policy of bombing the homes of Hamas leaders is against international law.
"While they may be targeting senior members of the factions and military wings, these organisations say there is no doubt that there are families there and they are in residential neighbourhoods," he said.
"As we have seen in this particular strike, it was a direct hit in the heart of the Jabaliya camp, the most densely populated in Gaza, home to 70,000 Palestinians."
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| Rayyan is the most senior Hamas official killed in the current Israeli offensive |
Israel says its assault on Gaza is aimed at ending persistent Hamas rocket attacks from the enclave, but its offensive has sparked international condemnation and protests around the world.
In Jerusalem on Thursday, a coalition of left-wing parties and peace groups voiced their opposition to the raids with a protest in front of the Israeli prime minister's home.
The protesters called for an immediate end to the assault, saying the escalation of violence was a disaster for both sides.
Meanwhile Tayyip Erdogan, Turkey's prime minister, has called for an immediate ceasefire from both sides.
"Our call to Israel now is to halt its fire and to the other side to stop firing rockets and other attacks," he said after talks with Egypt's president in the Red Sea resort of Sharm el-Sheikh.
Desperate Gazans
With Israel continuing to pound Gaza, the situation for Gaza residents is becoming increasingly desperate.
Most of the 1.5 million people in the densely populated enclave have no means of sheltering from the raids, and humanitarian groups say supplies of food and fuel are running dangerously low.
Hospitals have also reported shortages of even the most basic medicines and say they have no more capacity to deal with the growing numbers of casualties.
On Thursday, however, Tzipi Livni, the Israeli foreign minister, denied suggestions there was a humanitarian crisis in the Strip, adding "and therefore there is no need for a humanitarian truce".
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| Israeli tanks and troops have been massing at the Gaza border [AFP] |
Livni was speaking in Paris after talks with the Nicolas Sarkozy, the French president, whose call for a 48-hour ceasefire to allow in humanitarian aid has been rejected by Israel.
"Israel has been supplying comprehensive humanitarian aid to the Strip ... and has even been stepping this up by the day," the Israeli foreign ministry quoted Livni as saying.
However, Karen Abu Zayed, the commissioner for the UN Relief and Works Agency (UNRWA) in Gaza, said that in eight years of working in Gaza the need for aid had "never been so acute".
"I am appalled and saddened when I see the suffering around me," she said, adding the UNRWA has made an emergency appeal for $34m to help the Gaza population.
Hasan Khalaf, Gaza's assistant deputy health minister, described the ongoing assault on Gaza as "an Israeli massacre".
"There is no comparison between what we have and what [Israel] are doing to us. The international community are standing unable to help us, and yet we know they have been helping Israel for tens of years.
"Even now they are comparing those getting scared in the south of Israel, and those buried under the rubble after having their houses bombarded."