Rockets flew between Israel and Hamas as tensions in the region continue their upward climb.
Hamas security sources reported at least 60 Israeli airstrikes across Gaza on Monday night into Tuesday, including from F-16s, Apache helicopters and drones. The sources said at least 10 people were injured.
"Operation Protective Edge is underway. Targeting #Hamas capabilities that are terrorizing #Israel," Israel Defense Forces spokesman Lt. Col. Peter Lerner tweeted.
The position of the IDF has changed, the spokesman said at an earlier news briefing. Last week they were focused on de-escalation, but now Israel is preparing for a possible deterioration of the situation, he said.
Lerner 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 2012, 30,000 reservists were called up.
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 found to be alive and in critical condition.
"The enemy has crossed the red lines and will be made to pay the price for its crimes," Mushir Al-Masri, a Hamas leadership figure and member of the Palestinian parliament, wrote on his Facebook page. "The blood of our martyrs is precious ... and is fuel for the intifada and the resistance."
After that statement, 70 rockets were fired from Gaza into Israel, with Hamas claiming responsibility.
Israeli air raid sirens were heard near Yavne in central Israel as militants in Gaza stepped up their attacks. At least one person was wounded by shrapnel in Ashdod, according to Israeli rescuer services.
In another tweet, Lerner said that all summer camps, kindergartens and schools within a roughly 25 mile (40 kilometer) radius of Gaza have been forbidden because of the threat of rockets.
The escalation of military action comes after the slaying of a Palestinian teenager, which was perhaps an act of retaliation for the killing of three Israeli teens earlier.
Israeli police have questioned six suspects about what they did "before and during the murder" of the Palestinian teen who was abducted and burned to death, Israeli police spokesman Micky Rosenfeld said.
Naftali Schwartzburger, the lawyer of one of the six suspects, said on CNN affiliate Israeli Channel 2 on Monday that Israeli police conducted re-enactments of the killing of Mohammed Abu Khedair.
The fallout from the slayings might damage Israeli Prime Minister Benjamin Netanyahu's political future.
Foreign Minister Avigdor Lieberman, head of Yisrael Beiteinu party, said in a press conference Monday that he told Netanyahu of his intention to dissolve his party's joint faction with Netanyahu's Likud party, saying it was "not working." Lieberman criticized Netanyahu's handling of Gaza.
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 death of 16-year-old Abu Khedair, Netanyahu talked by phone with 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 said there is a "strong indication" the attackers may have been motivated by a desire for revenge over the deaths of the three Israeli teenagers, whose bodies were found a week ago in a field in the West Bank.
"Israel stands thoroughly against this. We promised to investigate. We promised to bring the perpetrators of that crime to justice and guess what? We delivered. We've done exactly that," said Mark Regev, spokesman for Netanyahu.
The teen's father said later Monday that he did not recall speaking with the Prime Minister.
"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."
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.
Israeli police also announced Sunday a confession in the killing of a Jewish Israeli 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, according to Rosenfeld.
'Stop this disgusting cycle of violence'
Meanwhile, the Abu Khedair family is irate over the treatment by Israeli police of one of the burning victim's cousins, an American high school student.
Relatives say Tariq Abu Khdeir, a 15-year-old high school sophomore from Florida, was beaten while being detained amid protests over his cousin's death. (The American branch of the family uses a different spelling of the family surname.)
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 said Monday, speaking from Florida.
Senior State Department officials told CNN that the United States was instrumental in securing the release of the youth.
Senior officials in the Obama administration said they were shocked at two videos in which Khdeir was seen being held down and pummeled by men in the uniform of Israeli security forces, the officials said.
The officials said they expect Tariq will be able to return home to Florida with his family in the next few weeks.
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."
But State Department officials questioned Dermer's remarks.
"They are investigating whether he was throwing rocks," another senior State Department official said. "Even if he was, the question is, was this the right response to that? Obviously we don't think so."
Regev, Netanyahu's spokesman, agreed.
"This is no excuse for this sort of behavior and we're currently investigating it. It's not the police investigating themselves. We've initiated an impartial, objective, independent inquiry into exactly what happened," he said.
'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.