Israel warned Wednesday night that it would hit back hard at Gaza terror groups, after Islamic Jihad fired more than 50 rockets at southern Israel in the heaviest attack since 2012.

Although the rocket fire subsided late in the evening, Israel raised its security alert in the south, and residents in southern Israel were told to stay within 15 seconds of rooms reinforced against rockets, amid indications that the dramatically escalated new round of confrontation was not over.

At least two rockets fell in residential areas, one of them causing light damage in Sderot. There were no direct injuries, but a Sderot woman, aged 57, was lightly hurt when running for cover.

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“If there is no quiet in our south, no quiet for the residents of Israel, there will be noise, lots of noise in Gaza,” Prime Minister Benjamin Netanyahu said at a joint press conference with Britain’s Prime Minister David Cameron. “And that’s putting it mildly.”

Israel later closed all border crossings into the Strip and Defense Minister Moshe Ya’alon canceled all visitation rights for Palestinian prisoners held in Israeli jails, apparently a punitive measure.

Earlier, Ya’alon said Israel would not allow the attacks to pass without a response. “We will not let Islamic Jihad or anybody else in Gaza disrupt the life of Israel’s citizens,” Ya’alon said. “When it’s not quiet in southern Israel, it won’t be quiet in Gaza — so that the terrorists will regret their rocket fire.” Ya’alon said Israel held Hamas responsible for the rocket fire since it rules Gaza, “and if it can’t ensure quiet it will pay a price.”

The IDF’s Southern Command chief, Sami Turgeman, said the army had “no intention of showing restraint.”

Security chiefs held emergency consultations in early evening, and then presented the political leadership with “a range of options,” military sources said.

Foreign Minister Avigdor Liberman demanded that Israel re-invade Gaza, from which it withdrew unilaterally in 2005, but Netanyahu did not back the idea when asked about it at his press conference with Cameron.

Cameron condemned the rocket attacks and earlier, during talks with President Shimon Peres, called them barbaric. “Let me be absolutely clear about these attacks from Gaza, we condemn them completely,” Cameron said. “They are a reminder once again of the importance of maintaining and securing Israel’s future and the security threats you face, and you have Britain’s support in facing those security threats.”

The visiting British prime minister noted that “these attacks are completely indiscriminate, aimed at civilian populations and that is a demonstration of how barbaric they are.” And he stressed that “there is no violent route to statehood” for the Palestinians.

Peres said Israel could not “behave as though nothing happened, and I’m sure the government will take the necessary steps to stop it. Hamas must understand that… we shall not accept Gaza as a center of terror, we cannot permit it. It’s not just a matter of wounded or killed; it’s a million and a half people who won’t have security or a night’s sleep. We’d clearly like to have peace but we must stop the terror.”

In Gaza, Hamas officials said they would hold Israel responsible for any “escalation,” and that IDF action would be met with fierce resistance.

“We hold the occupation responsible, we warn of the consequences of any escalation and we reiterate that resistance is the right of the Palestinian people to defend itself,” Ihab al-Ghassin, a Hamas spokesman, said.

Dozens of rockets as well as mortar rounds were fired from the Gaza Strip at southern Israeli towns on Wednesday afternoon. In an initial response hours later, the IDF fired tank shells at what it termed “terror targets” in the Strip

Israel put the number of rockets at over 60, and Islamic Jihad, which took responsibility for the attacks, said it had fired 90 rockets. The organization said in a statement that it had launched an ongoing “military operation,” which it termed “Breaking the Silence.”

No Israeli casualties were reported in what was the largest attack from the Strip since Operation Pillar of Defense in late 2012.

Most of the rockets were reported to have fallen in open areas, though at least one rocket landed in a residential neighborhood in Sderot. Damage was reported at two impact sites.

IAF jets were reportedly flying over the Strip, apparently in an effort to thwart further rocket launches, and tanks fired shells at several targets in the coastal territory.

Palestinian security agencies in the Strip evacuated their headquarters for fear of Israeli reprisal, Sky News reported.

Islamic Jihad indicated that the attack was in retaliation for the Tuesday killing at the hands of the IDF of three Islamic Jihad operatives as they prepared to fire at Israel from the Gaza Strip.

“The Al-Quds Brigades responded to (Israeli) aggression with a volley of rockets,” the group said.

Immediately following the rocket attack, Netanyahu said Israel would forcefully respond to any threat on its citizens.

“It seems that the rocket fire came in response to our counter-terrorism operations yesterday,” Netanyahu said. “We will continue to thwart and harm those who wish to harm us, and we will act against them with great intensity.

“Last year, the number of rockets fired from Gaza was the lowest in a decade, but we will not settle for that. We will continue to ensure the security of Israeli citizens in the south and throughout the country.”

Last week, Israel intercepted what it said was an Iranian shipment of rockets intended for terror groups in Gaza. Israeli military sources said the arms, including 409 rockets, were destined for Islamic Jihad.

AFP and Avi Issacharoff contributed to this report.