Nine Palestinian militants have been killed in Israeli air raids on Gaza following rocket attacks on Israel.
Hamas said six members of its military wing died in a strike near Rafah, in the south. Three others died in separate air strikes in response to rocket and mortar fire on Israel.
Later, a rocket struck near the southern Israeli city of Beersheba.
Tensions in the region are high following the murder of Palestinian teenager Mohammed Abu Khdair last week.
On Sunday, police said they had arrested six Jewish suspects in connection with the killing. Police told the BBC the 16-year-old was apparently "murdered because of his nationality". Details have not been divulged because the case is subject to a gagging order.
Mohammed Abu Khdair's killing followed the murder of three Israeli teenagers in the occupied West Bank, whose bodies were found a week ago.
Israel says two members of Hamas abducted and killed the youths, but it has denied any involvement.
Call for revenge
Emergency services in Gaza confirmed only two deaths from the air strike in Rafah but four others are believed to be buried under the collapsed site. A seventh victim was reported killed in another attack.
Earlier, two other Palestinian militants were killed in a separate air attack near a refugee camp in central Gaza.
The Israeli military said the strikes were in response to at least 25 rocket and mortar attacks on Israel on Sunday. It said the air raids targeted "terror" sites and concealed rocket launchers.
But Hamas's military wing, the Izzedine al-Qassam Brigades, vowed that Israel would pay "a tremendous price".
On Monday morning, the Israeli military said a rocket had struck the Beersheba area, 50km (31 miles) from Gaza.
An Israeli military patrol had also been attacked near the security fence along the border with Gaza, apparently with an anti-tank missile and fire-arms, it added.
The BBC's Kevin Connolly in Jerusalem says Israel has arrested hundreds of members of Hamas during its search for the kidnappers of the three Israeli teenagers.
That may help to explain the sudden intensification of rocket fire from Gaza, he says.
Israeli Prime Minister Benjamin Netanyahu has promised to bring to justice the killers of Mohammed Abu Khdair.
Mr Netanyahu's office said on Monday that he had spoken with the teenager's father, Hussein, offering his condolences and expressing his outrage at "the reprehensible murder".
"We acted immediately to apprehend the murderers. We will bring them to trial and they will be dealt with to the fullest extent of the law," it quoted him as telling Mr Abu Khdair.
"We denounce all brutal behaviour. The murder of your son is abhorrent and cannot be countenanced by any human being," Mr Netanyahu said.
Mohammed Abu Khdair was abducted on Wednesday morning and found dead hours later. He had reportedly been burnt to death.
His funeral in East Jerusalem on Friday triggered repeated clashes between police and Palestinian youths there and in Arab-Israeli towns in the north of Israel.
Earlier in the week, the three Israeli teenagers Naftali Frenkel, Gilad Shaer - both 16 years old - and Eyal Yifrach, 19 were buried, their funerals attended by thousands of people, including Mr Netanyahu. They were abducted while hitch-hiking in the West Bank and their bodies were found last Monday.
Justice Minister Tzipi Livni said her department was investigating some of the anti-Arab incitement seen on social media last week.
"These things need to be cut when they are small," she told Channel 2 TV. "At this moment, everybody's job should be to lower the flames."
Israeli officials said the six suspects remained in custody and were being interrogated.
They were described as young males, including some minors, all of whom live in the Jerusalem area.
About 50 people have been arrested in protests following Mohammed Abu Khdair's death, officials say.
Among them was the victim's 15-year-old cousin - a US citizen - whose family says he was beaten by Israeli security forces.
The boy, Tariq Khdair, who attends a school in Florida, was bailed after appearing in court on Sunday, accused of attacking police officers during the unrest. His parents deny that he was involved in the riots.