Palestinians have fired rockets into Israel and Israeli aircraft struck targets in the Gaza Strip in the heaviest exchange of fire since both agreed to an ceasefire in November.
No casualties were reported, but the violence threatened to shatter the calm that has prevailed for more than four months and prompted Israel's new defence 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, Mr Yaalon said he blamed the Islamic militant group Hamas, which has ruled Gaza since 2007.
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, 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.
Under the ceasefire 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.
A 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 cancer on Tuesday. They have blamed Israel for the death, saying he was not given proper medical care. 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.
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 his death to "resume popular protests."
In a separate development, Mr Yaalon warned battling forces in Syria that Israel would respond to any cross-border provocations. 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.