JERUSALEM (CNN) -- Eight Palestinians were killed in Israeli airstrikes and rocket attacks on Sunday and Monday, causing a leader of Hamas to warn that Israel "will be made to pay the price for its crimes."
The violence ramped up following the kidnapping of a 16-year-old Palestinian who was burned alive, itself a possible act of retaliation for the deaths of three Israeli teenagers. Israeli police arrested several people in the killing of the Palestinian teen.
Eight Palestinian militants were killed in more than 25 Israeli airstrikes and 39 rocket attacks across Gaza on Sunday night into Monday, said Ashraf Al-Qidra, a spokesman for the Health Ministry in Gaza. At least 15 people were injured, he said.
CNN originally reported nine killed but the death toll was lowered because one man believed to have been killed in a tunnel in Rafah was actually found to be alive and in critical condition.
Mushir Al-Masri, a Hamas leadership figure and member of the Palestinian parliament, expressed anger over the deaths.
"The enemy has crossed the red lines and will be made to pay the price for its crimes," he wrote on his Facebook page. "The blood of our martyrs is precious ... and is fuel for the intifada and the resistance."
Israel Defense Forces spokesman Lt. Col. Peter Lerner said an Israeli aircraft struck the tunnel in the southern Gaza Strip. He said Hamas constructed the tunnel for terror attacks against Israel civilians and soldiers.
At a Monday news briefing, Lerner said the position of the IDF has changed. Last week they were focused on de-escalation but now "we will respond to quiet with quiet," he said.
He said the IDF has already called up several hundred reservists and is prepared to add 1,500 more. During the last offensive on Gaza in November 2013, 30,000 reservists were called up.
An "abhorrent" murder
Israeli Prime Minister Benjamin Netanyahu on Monday spoke with the father of the Palestinian teenager who was burned alive in Jerusalem last week, expressing shock at what he called an "abhorrent" murder.
As anger continues to boil over the killing of 16-year-old Mohammed Abu Khedair, Netanyahu spoke by phone with the father, Hussein Abu Khedair, telling him that the killers will be brought to trial and "will be dealt with to the fullest extent of the law."
Police have arrested six Israeli Jewish suspects in connection with the teen's death. They said there was a "strong indication" that the attackers may have been motivated by a desire for revenge over the deaths of three Israeli teenagers, whose bodies were found a week ago in a field in the West Bank.
The teen's father told CNN later Monday that he did not recall speaking with Netanyahu.
"I was up until 5 a.m. this morning and tried to go to sleep around then," Hussein Abu Khedair said. "Around 7 or 7:30, I started getting phone calls from many Hebrew speaking people. I was tired and could not make out what or who I spoke to. I can't recall speaking to Netanyahu."
And Israeli police announced Sunday a confession in the killing of an Israeli Jewish teen a month ago. Shelly Dadon, 19, was kidnapped and stabbed to death by an Arab Israeli taxi driver, who has now provided details of the killing, , Israeli police spokesman Micky Rosenfeld said in a Tweet.
In the Knesset, Israel's parliament, lawmakers unanimously condemned the deaths of all five young people -- the four Israeli Jews and the Palestinian teen, The Jerusalem Post reported.
In a region that has experienced decades of fighting and mistrust, the past week's events have still managed to shock -- and to further embitter relations between Israelis and Palestinians.
"The murder of your son is abhorrent and cannot be countenanced by any human being," Netanyahu told the teen's father, according to the prime minister's office.
'Stop this disgusting cycle of violence'
The family is also irate over the treatment by Israeli police of one of the victim's cousins, an American high school student.
Relatives say Tariq Abu Khdeir, a 15-year-old high school sophomore from Florida, was brutally beaten while being detained amid protests over his cousin's death.
Khdeir, who was in Jerusalem to visit family during his summer vacation, was released on bail Sunday. The magistrate court ordered that he stay under house arrest for nine days at a relative's house in a different neighborhood from the family home.
"We're extremely concerned that he's under house arrest right now without really facing any legitimate charges that have been made public, and that those who beat him apparently are walking free," his family attorney told CNN on Monday, speaking from Florida.
Israeli authorities say Khdeir was part of a group of youths who attacked police.
"From what I understand about the facts of the case, this
is not just an innocent bystander who was pulled off of a schoolyard," Israel's ambassador to the United States, Ron Dermer, told "Fox News Sunday." "He was with six other people. They were masked. They threw petrol bombs and Molotov cocktails at our police. Three of them had knives. ... That does not excuse any excessive use of force, and our Justice Ministry is opening an investigation."
Khdeir says he was standing around, watching protesters, when he was accosted.
"I was attacked by police. I woke up in the hospital," he said as he walked out of court in Jerusalem on Sunday, sporting two black eyes and a swollen lip.
"I just saw somebody running at me, so I tried to run away," he told reporters.
Khdeir said he didn't know why he was attacked.
His relatives say he is the young man who appears in two separate videos being held down with his hands bound and beaten by men wearing the uniform of Israeli security forces.
The U.S. State Department has called for "a speedy, transparent and credible investigation and full accountability for the apparent excessive use of force."
Khdeir's mother says the family plans to sue Israeli authorities.
"He's doing a lot better," family lawyer Hassan Shibly told CNN's "New Day." "We need, really, all sides right now to just stop this disgusting cycle of violence where innocent children, both on the Israeli and Palestinian side, are being killed and injured almost daily nowadays."
In recent days, Israelis have rallied against racism and called for an end to the violence.
Netanyahu has called on Palestinian and other Arab leaders to condemn the violence as he has.
"We are different from our neighbors. There, the murderers are accepted as heroes and squares are named after them," he said Sunday, adding that Israel prosecutes instigators while the Palestinian leadership incites violence through media and the education system.
"We don't differ between terror and terror, and against both we will act harshly. And I don't differ between incitement and incitement in the state of Israel. ... We will not let extremists -- it doesn't matter from which side -- bring bloodshed to the area."
The protests at which he was detained were part of days of unrest in Jerusalem -- prompted by the killing of Abu Khedair -- during which Palestinian youths repeatedly clashed with Israeli security forces.
The clashes are the worst the city has seen in a decade.
'Demolish their houses'
Abu Khedair's mother, Suha, said she wants equal justice for the people who abducted him in the early morning as he was heading from his home to a mosque for prayers. He died after being burned alive and hit on the head with a blunt object, authorities say.
'If they sentence them and demolish their houses and give them life sentences, it might satisfy me a little," Suha Abu Khedair said.
The Israeli military destroyed the homes of the two main suspects in the killings of the three Israeli teenagers. Those suspects are still at large.
Palestinian Authority President Mahmoud Abbas has called on the United Nations to set up an international investigation into recent crimes against Palestinian people, including the killing of Abu Khedair, the Palestinian state news agency WAFA reported.
Officials from both sides have called for restraint amid fears that the cycle of horrific violence could continue.
Crackdown on Hamas
Israeli forces have also continued to crack down on the West Bank operations of Hamas, the militant Islamic movement that it accuses of being behind the killings of the three Israeli teenagers.
Hamas has praised the abductions of the three Jewish youths but denied responsibility for what happened.
Netanyahu vowed last week that "Hamas will pay" for the deaths of Eyal Yifrach, 19; Gilad Shaar, 16; and Naftali Frankel, 16, who was also a U.S. citizen.
The crackdown, which began after the teens were kidnapped on June 13, has involved hundreds of arrests, widespread searches of homes and deadly clashes.
CNN's Ben Wedeman and Kareem Khadder reported from Jerusalem; CNN's Jethro Mullen reported and wrote from Hong Kong. CNN's Michael Schwartz, Diana Magnay, Holly Yan, Josh Levs, Steve Almasy and Talal Abu Rahma contributed to this report.
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