Israel launched an offensive against Hamas in Gaza on Tuesday, bombing some 50 targets, including homes.

Israeli raids were reported throughout the Gaza Strip, including Gaza City, with jets attacking empty training camps belonging to Palestinian fighting groups.

Among the 50 sites the Israeli army said it targeted early Tuesday were four houses belonging to militants, three militant compounds, 18 concealed rocket launchers and other militant infrastructure sites. Most were targeted in airstrikes, and three were attacked from the sea.

Palestinian officials said more than 30 of the sites were bombed in little more than an hour before dawn, including two homes in southern Gaza, one of which was identified by a neighbor as belonging to a Hamas member.

A Twitter statement from the Israeli army said the offensive, dubbed "Operation Protective Edge," is intended to "stop the terror Israel's citizens face on a daily basis."

Gaza health official Ashraf Al-Kedra said at least nine Palestinian civilians were brought to a Gaza hospital with light to moderate injuries from the airstrikes, including several who suffered from shock. He said some of the injured Palestinians were treated and released.

There were no other reported casualties possibly because the buildings were believed to have been evacuated beforehand.

Israeli army spokesman Lt. Col. Peter Lerner said "We don't expect it to be a short mission on our behalf."

He added that the army will gradually increase its attacks on Hamas in Gaza, and is recruiting additional reservists for a potential ground invasion of Gaza.

Hamas has amassed about 10,000 rockets, including longer-range rockets that can reach "up to Tel Aviv and beyond," Lerner said, adding that the army was preparing for the possibility that Hamas would launch rockets toward Israel's heartland and its commercial and cultural hub.

Reflecting growing U.S. concern over the violence rapidly spiraling out of control, President Barack Obama called for Israelis and Palestinians alike to show restraint and put an end to acts of retribution, in some of his first public comments on the matter since the murder of three Israeli teenagers who went missing on June 12 touched off a new round of deepening mistrust and violence.

In an op-ed published in the Israeli newspaper Haaretz, Obama called it a "dangerous moment" for the region where a U.S. peace effort recently collapsed. Writing in emotional terms, he said he couldn't imagine the pain suffered by the parents of the three Israeli teens, but was also heartbroken by the senseless murder of a Palestinian teenager who many suspect was killed in revenge.

"All parties must protect the innocent and act with reasonableness and restraint, not vengeance and retribution," Obama said.

Obama's comments — published in Hebrew, Arabic and English — appeared less than two days after Israeli airstrikes killed at least nine Palestinian fighters and injured four — including a 4-year-old child — in the deadliest exchange of fire since the latest round of attacks began weeks ago. The assualt was followed by a return of missile fire that saw around 100 rockets fired into Israeli territory, according to Hamas.

Hamas' armed wing claimed responsibility for scores of missiles fired Monday at the Israeli cities of Ashdod, Ofakim, Ashkelon and Netivot. Israel's Iron Dome defense system intercepted about a dozen rockets. One person was lightly wounded by shrapnel in Ashdod, Israeli news website Haaretz reported.