JERUSALEM -- Palestinian militants launched several rockets into southern Israel, as Israeli aircraft struck targets in the Gaza Strip early Wednesday in the heaviest exchange of fire between the sides since they agreed to an internationally brokered cease-fire in November.
There were no casualties reported, but the violence nonetheless threatened to shatter the calm that has prevailed for more than four months and prompted Israel's new defense minister to warn that the Jewish state will not sit back if militants attack the south of the country.
"We will not allow shooting of any sort (even sporadic) toward our citizens and our forces," Moshe Yaalon, a former military chief of staff, said in a statement.
Although there was no claim of responsibility for the rockets fired from Gaza early Wednesday, Yaalon said he holds the Islamic militant group Hamas, which has ruled Gaza since 2007, responsible for all such attacks from the seaside strip.
Israel launched an offensive against Hamas last November in response to an increase in rocket fire out of Gaza. During eight days of fighting, Israel carried out hundreds of airstrikes in Gaza, while Gaza militants fired hundreds of rockets into Israel. More than 160 Palestinians, including dozens of civilians, and six Israelis were killed in the fighting before Egypt brokered a truce.
In recent weeks, there have been a handful of rocket attacks, including one that took place as President Barack Obama was visiting Israel two weeks ago. Overnight Wednesday, Israel responded for the first time by striking a pair of empty fields in northern and eastern Gaza.
But just around the time Yaalon was speaking on Wednesday morning, two more rockets exploded in the Israeli border town of Sderot, according to police. Air raid sirens sounded throughout the town, forcing people on their way to work and school to take cover. No injuries were reported.
The Israeli military said a total of five rockets had been fired in the past 24 hours, including two that exploded prematurely inside Gaza.
Under the cease-fire, Israel pledged to halt its policy of attacking militant leaders and to ease a blockade it has imposed on Gaza since the Hamas takeover in 2007. Hamas pledged to halt rocket attacks on Israel. A number of smaller militant groups also operate in Gaza, including groups that draw inspiration from the al-Qaida global terror network.
U.N. Mideast envoy Robert Serry appealed for calm in a statement, saying he is "worried" tensions could threaten the informal truce. "It is of paramount importance to refrain from violence in this tense atmosphere and for parties to work constructively in addressing the underlying issues," he said.
Ihab Ghussein, the Hamas government spokesman, accused Israel of using the airstrikes to "divert the attention" from unrest in Israeli prisons. "They think that through escalation on Gaza front they can hide the truth," he said, and urged Egypt, the guarantor of the cease-fire, to intervene.
Palestinian prisoners have been rioting and hunger striking since a 64-year-old prisoner died of throat cancer on Tuesday. The Palestinians have blamed Israel for the man's death, saying he was not given proper medical care. The prisoner, Maysara Abu Hamdiyeh, had been serving a life sentence for his role in a foiled attempt to bomb a busy cafe in Jerusalem in 2002.
Israel's chief military spokesman, Brig. Gen. Yoav Mordechai, said the army was on alert for "riots" in the West Bank ahead of an autopsy planned later in the day. He accused the Palestinian Authority, which governs in the West Bank, of exploiting the death to "resume popular protests."
At protests across the West Bank Wednesday afternoon, hundreds of Palestinians threw rocks and rolled burning tires at soldiers, prompting a response with tear gas, the Israeli military said.
In Ramallah, protesters waved pictures of the man that succumbed to cancer and chanted "with our souls and blood we will redeem the prisoner."
Prisons Authority spokeswoman Sivan Weizman said Abu Hamdiyeh was treated by Israeli specialists and died in a hospital in Beersheba. She said the prison service asked the parole board to release Abu Hamdiyeh last week after his cancer was diagnosed as terminal last, but the appeal was still being processed at the time of his death.
Weizman said almost all of the 4,600 Palestinian prisoners detained by Israel refused their breakfasts Wednesday morning in a symbolic act of protest.
As news of Abu Hamdiyeh's death spread Tuesday, Palestinian prisoners in several jails began banging on their cell doors and hurling objects. Later, protests spread to Jerusalem and parts of the West Bank where protesters hurled fire bombs and rocks at soldiers.
Tensions are high in Israeli lockups, where thousands of Palestinian security prisoners are being held. Some have staged hunger strikes and Palestinians have held large protests demanding their release.
After decades of conflict with Israel, the issue of prisoners is emotionally charged in Palestinian society. The Palestinians revere the prisoners as standing up to Israeli occupation. In Israel, the prisoners, who are serving time for crimes ranging from stone throwing to mass murder, are seen as terrorists.
In a separate development, Israel's defense minister issued a tough warning to battling forces in Syria, saying Israel would respond to any cross-border provocations.
On Tuesday, the Israeli military said a mortar shell exploded on its side of the frontier in the Golan Heights. The military said its soldiers returned the fire and said it scored a direct hit. Mortar shells and machine gun fire have sporadically hit Israeli territory in the Israeli-controlled Golan Heights since Syria descended into civil war following its March 2011 uprising.
"Israel has no intention of ignoring fire from Syria toward Israeli territory, incidental or not, and will respond with a firm hand," Yaalon said. "As far as we are concerned, the Syrian regime is to be held responsible for everything happening in its territory."
Israel, which has warily watched the fighting in Syria raging close to its frontier, is concerned that some of the al-Qaida affiliated groups fighting alongside the rebels against the Syrian government forces could set their sights on Israel when the civil war ends.
Syria has kept its frontier with Israel quiet for most of the past 40 years, even while providing support and refugee for Israel's bitterest enemies, including the Iranian-backed Hezbollah.