JERUSALEM (AP) -- The Israeli army said it launched an offensive operation early Tuesday against the Gaza Strip to quell rocket attacks, and a Palestinian official said Israeli airstrikes injured at least nine Palestinians.
The Israeli airstrikes come after Gaza militants fired dozens of rockets at southern Israel on Monday, setting off air raid sirens and forcing hundreds of thousands of Israelis to stay indoors.
The military rushed more forces to the border late Monday and had warned that such an offensive was likely.
A Twitter statement from the Israeli army said the offensive, dubbed "Operation Protective Edge," is intended to "stop the terror Israel's citizens face on a daily basis."
The army said it was carrying out airstrikes in the Gaza Strip early Tuesday. It did not elaborate.
Gaza health official Ashraf Al-Kedra said at least nine Palestinian civilians were brought to a Gaza hospital with light to moderate injuries from the airstrikes, including several who suffered from shock. He said some of the injured Palestinians were treated and released.
The latest violence came as Israel pressed forward with its investigation of six Jewish youths suspected of abducting and killing a Palestinian teenager, and Israeli leaders sought to calm an emotional debate over whether the country's politically charged atmosphere led to the gruesome crime. An Israeli official said three of the youths had confessed to the attack.
Tensions have been high since three Israeli teenagers kidnapped June 12 in the West Bank were later found dead, followed by last week's slaying of the Palestinian youth in what many suspect was a revenge attack. Throughout the unrest, Gaza militants have launched more than 200 rockets and mortars into Israel, including close to 100 on Monday alone.
Israel has responded with dozens of airstrikes, but has not been able to halt the attacks. Eight Palestinian militants were killed in fighting Monday, the highest death toll yet.
The army said at least 70 rockets were fired at Israel from Gaza on Monday, including 40 launched in a single hour after nightfall, setting off air raid sirens up to 50 miles (80 kilometers) from Gaza, the military said.
Twelve rockets were intercepted by rocket-defense batteries, it added, while the others landed in open areas. It was the deepest penetration of rocket strikes in the current round of fighting and raised the likelihood of an even tougher Israeli response.
The army later said that eight more rockets and mortars were fired at Israel from Gaza starting shortly before midnight and into early Tuesday, and an additional rocket was intercepted above the Israeli town of Sderot, close to the border with Gaza.
Maj. Gen. Yoav Mordechai, a senior military official, gave a special interview in Arabic to Al-Jazeera, warning that Hamas would bear the consequences for the escalation.
Among the dead were six Hamas militants who Israel said were killed in an accidental blast in a tunnel packed with explosives. Hamas, the Islamic militant group that controls Gaza, vowed revenge, saying "the enemy will pay a tremendous price."
Lt. Col. Peter Lerner, an Israel Defense Forces spokesman, had said the army was moving more infantry forces to the Gaza border and had received authorization to mobilize up to 1,500 reservists.
The deaths of the Hamas militants had made a "substantial influence" on the situation, he said.
"There is a potential of deterioration due to their death. Therefore the IDF has to continue to reinforce capabilities in the south, with the potential that things could escalate further," he said.
Israeli security officials had said late Monday that Israel was leaning against a massive operation and would likely increase the pressure gradually with stronger and more numerous retaliatory attacks. They spoke on condition of anonymity because they were not authorized to discuss internal military deliberations with reporters. Israeli Cabinet ministers voted in favor of more intense airstrikes, Channel 2 TV said.
In Washington on Monday, State Department spokeswoman Jen Psaki said the U.S. condemns the rocket fire. "We also support Israel's right to defend itself against these attacks," she said.
President Barack Obama called for Israelis and Palestinians alike to show restraint and put an end to acts of retribution, in some of his first public comments on the matter since the murder of three Israeli teenagers touched off a new round of violence and deepening mistrust.
In an op-ed published in the Israeli newspaper Haaretz, Obama called it a "dangerous moment" for the region where a vaunted U.S. peace effort recently collapsed. Writing in emotional terms, he said he couldn't imagine the pain suffered by the parents of the three Israeli teens, but was also heartbroken by the senseless murder of a Palestinian teenager who many suspect was killed in revenge.