Palestinian militants in Gaza fired a new wave of rockets that landed deep inside Israel Thursday, defying Israeli retaliatory attacks and threats.

As the violence threatened to escalate the day after a deadly Jerusalem bombing, Israel got a boost from the visiting U.S. defense chief, who said no country could tolerate the "repugnant" attacks on its soil.

Police said Gaza militants fired 10 rockets and mortars toward Israel Thursday, including two rockets that exploded north of the city of Ashdod, a main Mediterranean port city about 20 miles (30 kilometers) north of Gaza — a first since Israel and Gaza's Hamas rulers reached an unofficial truce following a three-week war that ended in January 2009. Israeli airstrikes hit a number of Gaza targets in retaliation throughout the day.

Neither side reported injuries or said they wanted a new fight. But the new hostilities could easily spin out of control, especially if civilian deaths mount.

Wednesday's bombing killed a British tourist, and five members of a Jewish family were slain while they slept in a West Bank settlement earlier this month. Israel has blamed Palestinians for both attacks.

Also this week, Israeli shelling killed three children and their uncle in Gaza. The army said it was targeting militants.

The fighting in Gaza has been the fiercest since Israel went to war there to try to curb years of rocket attacks. The fierce three-week offensive killed some 1,400 Palestinians, including hundreds of civilians. Thirteen Israelis also died. The volatile border has remained largely calm since.

Israel says Hamas has used the lull to rearm with longer distance rockets that can reach as far as Tel Aviv, about 30 miles (50 kilometers) from Gaza.

Defense Minister Ehud Barak blamed Hamas for the rocket fire and vowed to strike back.

"Israel will not tolerate these terror attacks and we will not allow terror to rise once again in the region," he said.

His tough stance was backed by visiting U.S. Defense Secretary Robert Gates, who said that no sovereign state could tolerate rockets fired at its people.

"Israel, like all nations, has the right to self-defense and to bring to justice the perpetrators of these repugnant attacks," he said.

Citing gag orders, Israeli security officials have said little about the investigations into Wednesday's bus stop bombing or the knife killings two weeks ago.

Officials identified the victim of the Jerusalem bombing as Mary Jean Gardner, a 59-year-old British tourist who had been taking courses at Jerusalem's Hebrew University. In Washington, the State Department said five of the wounded were Americans, one of whom remains hospitalized.

On Thursday President Barack Obama called Israeli Prime Minister Benjamin Netanyahu to offer condolences. The White House said Obama reaffirmed the U.S. commitment to Israel's security.

Police spokesman Micky Rosenfeld said Jerusalem and southern Israel remained on a heightened state of alert.

Israeli counterterrorism expert Boaz Ganor said the bombing and knifing attacks appeared to be individual initiatives, as opposed to the organized attacks by militant groups that Israel usually faces.

The former usually kill fewer people, but are more difficult to stop, he added.

"Israeli intelligence is quite good in thwarting suicide attacks," he said. "It may be less able to deal with local and personal attacks."

Israeli police officials, speaking on condition of anonymity because of the sensitive nature of the investigations, said that even if the attacks were individual acts, Israel believes Hamas guided and motivated the attackers.

Yitzhak Reiter, a Mideast expert at the Hebrew University, said Islamist groups are currently seeking an alternative to suicide bombings, which largely backfired in the last decade by turning world opinion against them.

Peace talks between Israel and Hamas' rival, Palestinian President Mahmoud Abbas, collapsed after the 2008 war, reviving only briefly for three weeks in September 2010.

Abbas, who rules on in the West Bank, has rejected violence and condemned Wednesday's bombing.

Hamas, which violently wrested control of Gaza from Abbas loyalists in June 2007, sees the diplomatic standstill as proof that only an armed struggle will win the Palestinians a state.