Six Americans were injured in the deadly Jerusalem bus bombing Wednesday, a State Department official tells Fox News.

A British tourist was killed when a bomb went off on a bus during evening rush hour in Jerusalem.

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 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 aircraft struck the Gaza Strip on Thursday in response to militant 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.

Militants operating near the Israeli border sent at least nine rockets and mortars -- including two rockets that reached the city of Ashdod -- flying at Israeli communities throughout the day, drawing an Israeli airstrike in retaliation. No injuries were reported on either side.

U.S. Defense Secretary Robert Gates called the rocket attacks "repugnant" and defended the Israeli attacks in the Gaza Strip as legitimate self-defense.

He spoke during a meeting with his Israeli counterpart, Ehud Barak, who said that "Israel will not tolerate these terror attacks and we will not allow terror to rise once again in the region."

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.

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.

Israeli security officials have said little about the investigations into Wednesday's bus stop bombing and the knife killings of five members of a family in a Jewish settlement in the West Bank two weeks ago.

Officials identified the victim of the Jerusalem bombing Thursday as Mary Jean Gardner, a 59-year-old British tourist who had been taking courses at Jerusalem's Hebrew University.

Police spokesman Micky Rosenfeld said there were gag orders preventing discussion of either investigation. He said only that Jerusalem police remained on a heightened state of alert.

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.

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

With no signs that the two sides are making any progress to resume peacemaking, Abbas is pushing to get the international community to recognize a Palestinian state by fall in the absence of a peace deal. Such recognition at the United Nations would not change things on the ground, but could further isolate Israel.

Abbas, who controls only the West Bank, has repeatedly rejected violence and condemned Wednesday's bombing.

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

The Associated Press contributed to this report.