An Iron Dome air defense system fires to intercept a rocket from Gaza Strip in the costal city of Ashkelon, Israel, on Saturday, July 5, 2014.

JERUSALEM - Israel launched what could be a long-term offensive against the Hamas-ruled Gaza Strip on Tuesday, the military said, striking at least 50 sites in Gaza by air and sea and mobilizing troops for a possible ground invasion in order to quell rocket attacks on Israel.

The army said its offensive, dubbed "Operation Protective Edge," is aimed at striking Hamas and ending 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.

Nearly 300 rockets and mortars have been fired at Israel in recent weeks, including a barrage of about 80 projectiles on Monday alone, the army 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 army spokesman Lt. Col. Peter Lerner. "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 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.

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 to stay indoors, including Israelis in the major southern city of Beersheva, Lerner said.