At least two Grad rockets fired by Palestinian armed groups in Gaza on Thursday slammed into the port city of Ashdod and just north of it, police and the Israeli army said.
Medics said no one was injured in the strikes, which came a day after Gaza's Hamas rulers vowed to rein in recalcitrant armed groups who had vowed to hit ever-deeper into Israel.
Defense Minister Ehud Barak warned that the Jewish state had no choice but to respond.
"We have to respond," the stocky Barak said at a joint press conference with visiting U.S. Defense Secretary Robert Gates.
"Israel will not tolerate these terrorist attacks and we will not allow terror to rise once again," Barak said.
Police spokesman Micky Rosenfeld confirmed that two Grad rockets had landed in Israel, saying: "One landed in the city and the other landed north of the city."
An army spokeswoman confirmed two Grads had hit Ashdod.
Earlier, security sources told AFP one of the rockets landed in an area "just south of Rishon-le-Tzion" -- which is significantly further north, in an area around 15 kilometers (nine miles) from Tel Aviv.
Officials in the area had "heard a loud blast" but it later transpired the blast was not caused by the second rocket, which had landed just north of Ashdod.
The rocket fire came a day after Islamic Jihad's military wing, the al-Quds Brigades, fired three Grads at Beersheva and Ashdod and vowed to fire more at cities deep inside the Jewish state as it entered "a new phase" of resistance.
"Al-Quds Brigade has entered a new phase of bombing targets which are further away, where thousands of Israelis live," group spokesman Abu Ahmad told AFP.
Ashdod lies about 30 kilometers (18 miles) up the coast from Gaza, while Rishon-Le-Tzion is located another 25 kilometers (15 miles) further north.
Grad rockets are understood to have a range of up to 50 kilometers (30 miles).
Earlier on Thursday, Israeli aircraft struck the Gaza Strip in response to rocket and mortar attacks, stoking concerns that a grave new round of hostilities will fill the vacuum left by an impasse in Israeli-Palestinian peacemaking.
Two years of relative calm have been unraveling in recent weeks with acts of violence against Israelis - including a deadly bombing in Jerusalem on Wednesday that killed an unidentified female British tourist - and Israeli reprisals against Gaza armed groups, which in one case killed four Palestinian civilians.
Armed groups operating near the Israeli border sent at least seven rockets and mortars flying at Israeli communities throughout the day, drawing a pair of air attacks. No injuries were reported on either side.
Because of the superiority of Israel's military, its enemies consider hitting Israeli cities as the most effective strategy in a war against the Jewish state. At least two rockets this week have landed in Beersheba, the largest city in southern Israel.
Israel's inability to halt the rocket fire has drawn public criticism, especially since Israel has spent an estimated $200 million developing a system that is meant to shoot down the short-range rockets that militants use. Although officials have repeatedly promised the Iron Dome is ready to be deployed, it so far hasn't been used in the field.
Home Front Minister Matan Vilnai, who oversees civil defense, refused to discuss the holdup in an interview on Israel Radio Thursday.
The rising wave of violence has been the fiercest since Israel went to war in Hamas-ruled Gaza more than two years ago to try to curb years of frequent rocket attacks.
On Thursday, Israel filed a complaint with the U.N. about the rising wave of attacks on its citizens.
Before leaving for a brief trip to Russia, Israeli Prime Minister Benjamin Netanyahu warned militants not to test Israel's "iron will" and vowed a tough response. But he also expressed hope that calm would be restored.
Neither side is thought to be spoiling for a major fight, but the concern is that events will spiral out of control.
In late December 2008, Israel responded to months of intense rocket fire and years of more sporadic attacks with a fierce three-week offensive into Gaza, killing some 1,400 Palestinians, including hundreds of civilians. Thirteen Israelis also died in that offensive.
The violently anti-Israel Hamas movement that rules Gaza has largely honored an unofficial truce since then. But the lull has been fraying in recent weeks - and at a particularly fragile diplomatic moment.
Hamas, which violently wrested control of Gaza in fighting with President Mahmoud Abbas loyalists in June 2007, sees the diplomatic standstill as proof that only an armed struggle against Israel will win the Palestinians a state.