Hamas militants bombarded a major southern Israeli city with rocket fire on Friday, unleashing their most powerful weapons yet in a week of tit-for-tat fighting that threatens to destroy a five-month-old cease-fire.

Both Israel and Gaza's Hamas rulers held out hope the calm would be restored, but also vowed to strike hard at each other if violence persisted.

"If you want to leave the truce, we are ready. And if you want to continue it, then abide by it," said Hamas strongman Mahmoud Zahar said in a Friday sermon.

The Egyptian-mediated truce took effect last June, largely halting a cycle of Palestinian rocket attacks on southern Israel and deadly Israel reprisals.

The cease-fire has mostly held, but began to deteriorate last week after an Israeli military raid on what the army said was a tunnel that militants planned to use for a cross-border raid. Eleven militants have been killed, and some 140 rockets and mortars have been fired from Gaza at Israel. Israel also has shut Gaza's vital border crossings, blocking the entrance of food, humanitarian goods and fuel into the impoverished area.

Friday's rocket fire was among the heaviest yet. Nearly 20 rockets were fired into southern Israel, including four Grad-type Katyushas that landed in Ashkelon, some 17 miles north of Gaza. One person in a southern Israeli village was lightly wounded by shrapnel.

The foreign-made Katyushas are believed to be smuggled into Gaza and have longer ranges than the crude homemade rockets usually fired by militants. With 120,000 people, Ashkelon is the biggest population center in rocket range, and Israel has responded harshly to past attacks on the coastal city.

The barrage came in response to an earlier Israeli airstrike that wounded two militants as they attempted to fire rockets. Hamas said the longer-range rockets were meant to show Israel what could await it if the truce collapses.

It was the first time that rockets have reached Ashkelon in the current round of fighting. "The escalation of the Hamas reaction is that they want to send a message to Israel that they can't break the cease-fire without paying a price for it," said Mouin Rabbani, an independent analyst based in Jordan.

By sundown, the sides appeared to be pulling back, and the area settled into relative calm.

"We will keep protecting our soldiers and people and keep acting against attempts to interrupt the cease-fire, but if the other side will want or wish to keep the cease-fire alive, we'll consider it seriously," Israeli Defense Minister Ehud Barak said during a stop in southern Israel.

Even so, Israeli Prime Minister Ehud Olmert held an emergency meeting with Barak and other top security officials to discuss the situation. Afterward, Olmert indicated that the border crossings would remain closed and military action would continue if necessary.

"The government pressure on Hamas will continue in connection to the operations of the crossings and other means," Olmert said.

Israel controls all of Gaza's official cargo crossings. Its decision to close the crossings last week has led to severe shortages of basic goods, caused the United Nations to suspend food aid distribution to tens of thousands of people, and forced Gaza's only power plant to halt operations. The shutdown has led to blackouts in parts of Gaza, though other areas still receive power directly from Egypt and Israel.

About 750,000 people in Gaza — just over half the territory's population — rely on the United Nations for food. The sanctions drew criticism Friday from the European Union.

"I call on Israel to reopen the crossings for humanitarian and commercial flows, in particular food and medicines," EU External Relations Commissioner Benita Ferrero-Waldner said Friday. "Fuel deliveries for the Gaza power plant should be resumed immediately."

Hamas, an Islamic militant group that advocates Israel's destruction, has ruled Gaza since routing forces loyal to Palestinian President Mahmoud Abbas in June 2007. Israel continues to battle Hamas, while conducting peace talks with Abbas' government in the West Bank.

About 7,000 Hamas loyalists marched though central Gaza City following noon prayers, waving green flags and chanting slogans against Abbas' Fatah movement.

Hamas scuttled reconciliation talks last week, meant to take place in Egypt, accusing Fatah of imprisoning Hamas supporters in the West Bank.

Hamas supporters held a banner reading "History will not forgive collaborators," a reference to Fatah's relations with Israel.