Three Palestinians, including a schoolboy, were killed on Sunday, raising the death toll from Israeli air strikes on Gaza to 18 in less than 48 hours, medics said.
The latest raid hit an area northeast of Gaza City, killing a 60-year-old man who medics said was a civilian, several hours after a strike slightly further north killed a 12-year-old boy in Jabaliya refugee camp.
The spike in violence erupted on Friday afternoon when an Israeli strike killed the leader of the Popular Resistance Committees (PRC).
It sparked a bloody cross-border exchange in which 15 Gazans were killed and more than 100 rockets fired at Israel on Friday and Saturday, marking the deadliest 24-hour period in the border area in more than three years.
Since then, three more people have been killed, raising the toll to 18, including 15 militants, medics said. At least 30 people were wounded, six of whom were in critical condition.
The European Union and United States have urged both sides to restore calm.
But Palestinian militants vowed to avenge their dead and Israel threatened to hit back if its citizens came under renewed rocket attacks from the coastal enclave.
Since midnight, Israel staged at least four strikes, one of which killed a man in Zeitun in eastern Gaza City, and a second which killed 12-year-old Ayub Asaliya who was on his way to school in Jabaliya refugee camp.
His seven-year-old brother was moderately wounded, medics said.
Another strike in eastern Gaza City injured one person before the 60-year-old was killed northeast of the city.
By Saturday evening, the army said militants had fired 108 rockets and projectiles across the border, with 80 hitting Israel while another 28 were intercepted by its Iron Dome anti-missile system.
Another eight rockets have been lobbed at Israel since midnight, police said. Since Friday, four people have been wounded, Israeli medical sources said, with press reports identifying them as Thai agricultural workers.
Residents of southern Israel were on Sunday staying close to their bomb shelters, and schools were being kept shut across the region, the education ministry said.
The military, which has conducted at least 20 air strikes since Friday, says the aim is to halt rocket fire on southern Israel.
On Sunday, a spokesman for Israeli Prime Minister Benjamin Netanyahu said the air force had hit a site for firing long-range Iranian-built Fajr-5 missiles at Israel.
"The IAF destroyed launching pits of Fajr long range rockets that can hit Tel Aviv," Ofir Gendelman wrote in a posting on Twitter. "This is the result of meticulous intelligence work."
Of the dead, 10 were militants from the radical Islamic Jihad movement, while another five were from the PRC and included the group's leader, Zuhair al-Qaisi, who died in the first strike on Friday.
The army said Qaisi was involved in planning a deadly attack which killed eight in Israel's southern Negev desert last August by militants who sneaked across the border from Egypt's Sinai, and claimed he was planning a similar attack "in the coming days."
Israeli Defence Minister Ehud Barak said on Saturday he expected the violence to carry on for "another day or two," and Netanyahu said on Sunday that the air campaign would continue for "as long as necessary."
"We extracted a high price from them and will continue to do so," he said at the start of Israel's weekly cabinet meeting. "We will act as long as necessary."
Gaza's Hamas rulers, whose military wing has not been involved in the rocket fire, said on Saturday they were seeking Egyptian help to negotiate an end to the violence but that little progress had been made.
"Until now there has been nothing productive in terms of ending the aggression against Gaza and the aggression is continuing and so we affirm the right of all factions to defend their people," Hamas spokesman Sami Abu Zuhri told AFP.
But a spokesman for Islamic Jihad said it would not contemplate any form of ceasefire while Israel continued its deadly raids.
"If the Israeli aggression continues and there are more victims there will be no room for discussion about a calm. There's nothing new, the escalation continues, Israeli air strikes are continuing and we will deal with the escalation," spokesman Daud Shihab told AFP.