JERUSALEM (AP) — The Israeli military launched what could be a long-term offensive against the Hamas-ruled Gaza Strip on Tuesday striking at least 50 sites in Gaza by air and sea and mobilizing troops for a possible ground invasion aimed at stopping a recent barrage of rocket attacks against Israel.

The army said "Operation Protective Edge" looks to strike the Islamic Hamas group and end the rocket fire that has intensified in recent weeks amid tensions over the killing of three Israeli teenagers and the apparent revenge killing of a Palestinian teenager.

In a statement, the military said it was seeking to "retrieve stability to the residents of southern Israel, eliminate Hamas' capabilities and destroy terror infrastructure operating against the State of Israel and its civilians."

Nearly 300 rockets and mortars have been fired at Israel in recent weeks, including a barrage of about 80 projectiles on Monday alone, the military said. Israel has responded with dozens of airstrikes, and eight Palestinian militants were killed Monday. Israel had signaled that it would not launch a larger offensive if the militant group Hamas ceased the rocket fire.

"They chose the direction of escalation," said military spokesman Lt. Col. Peter Lerner, in a telephone briefing to reporters. "So the mission will go on as long as we feel it is necessary to carry it out. We don't expect it to be a short mission on our behalf."

Among the 50 sites the army said it targeted early Tuesday were four houses it said were "activity sites" belonging to Hamas militants involved in launching rockets at Israel or other militant activity. There were no reported casualties in the strikes.

The military identified the men whose houses were targeted as Eiad Sakik, Abdullah Hshash, Samer Abu Daka, and Hassan Abdullah. The Associated Press filmed Abu Daka and Abdullah's demolished homes in the Khan Younis area of the Gaza Strip. The state of the other homes could not be immediately confirmed.

In addition, the military said it struck three militant compounds, 18 concealed rocket launchers, and other militant infrastructure sites. Most were targeted in airstrikes, and three were attacked from the sea.

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.

Lerner, the military spokesman, said 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.

The army ordered hundreds of thousands of Israelis within a 40-kilometer (25-mile) radius of the Gaza Strip, including Israelis in the major southern city of Beersheva, to stay indoors and near shelters, Lerner said. Kindergartens and summer camps in the area were cancelled.

Lerner said last month's kidnapping and killing of three Israeli teenagers in the West Bank was connected to the intensified rocket fire carried out by Hamas militants in Gaza. Israel blames Hamas for the teens' abduction and is conducting a manhunt for two Hamas-affiliated Palestinians in the West Bank it believes carried out the kidnapping and killing.

The Israeli government has not yet provided proof of Hamas' involvement in the kidnapping.

"The abduction of the three boys only cost them, and they had no gains from it," Lerner said. "Therefore they have increased their involvement in rocket fire," adding that nearly all of the rocket fire at Israel Monday was carried out by Hamas, not militants affiliated with other groups. He said Hamas was "trying to gain clout."

Tensions have been high since three Israeli teenagers kidnapped June 12 in the West Bank were later found dead, followed by last week's slaying of the Palestinian youth in what many suspect was a revenge attack. Throughout the unrest, Gaza militants have launched more than 300 rockets and mortars into Israel, including close to 100 on Monday alone.

Twelve rockets were intercepted by rocket-defense batteries, the army said, while the others landed in open areas. Israel had responded with dozens of airstrikes, but could not halt the attacks. Eight Palestinian militants were killed in fighting Monday, the highest death toll yet.

Eight more rockets and mortars were fired at Israel from Gaza starting shortly before midnight and into early Tuesday, and an additional rocket was intercepted above the Israeli town of Sderot, close to the border with Gaza, the military said.