JERUSALEM // Palestinian militants launched several rockets into southern Israel as Israeli aircraft struck targets in the Gaza Strip early yesterday, in the heaviest exchange of fire between the sides since they agreed to an internationally brokered ceasefire in November.
No casualties were reported, but the violence threatened to shatter the calm that has prevailed for more than four months, prompting Israel's new defence minister to warn that the Jewish state would not sit back if militants attacked the south of the country.
"We will not allow shooting of any sort towards our citizens and our forces," said Moshe Yaalon, a former military chief of staff.
Although there was no claim of responsibility for the rockets fired from Gaza, Yaalon said he held the Islamist group Hamas, which has ruled Gaza since 2007, responsible for all such attacks from the area.
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 air strikes, 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 during a visit by the US president, Barack Obama, to Israel two weeks ago.
Yesterday, Israel responded for the first time by striking a pair of empty fields in northern and eastern Gaza.
But about the time Yaalon was speaking yesterday morning, two more rockets exploded in the Israeli border town of Sderot, police said.
Air raid sirens sounded throughout the town, forcing people on their way to work and school to take cover.
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 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.
Robert Serry, the Middle East envoy for the United Nations, appealed for calm, saying he was "worried" that 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.
In a separate development, Israel's defence minister issued a tough warning to battling forces in Syria, saying Israel would respond to 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 scored a direct hit. Mortar shells and machine gunfire have sporadically hit Israeli territory in the Israeli-controlled Golan Heights since Syria descended into civil war following the uprising in March 2011.
"Israel has no intention of ignoring fire from Syria toward Israeli territory, incidental or not, and will respond with a firm hand," said Mr Yaalon. "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 close to its frontier, is concerned that Al Qaeda-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 sanctuary for Israel's enemies, including the Hizbollah.
