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Palestinians inspect ruins in Khan Yunis after the Israeli airstrikes against Gaza on 8 July 2014.

Israel has launched what it described as an open-ended and escalating offensive against the Hamas-ruled Gaza Strip, as air strikes and naval gunfire hit 50 sites overnight.

As part of a new offensive dubbed "Operation Protective Edge", Israeli troops have been mobilised along the Gaza border and a limited number of reserves called up for a possible ground invasion.

The strikes came after Israeli army sources said troops were being put on notice of "preparation for escalation". The Guardian saw columns of tank and armoured personnel carriers moving along the main highway between Jersualem and Erez, on the Gaza border.

Rocket attacks from Gaza initially from Islamic factions other than Hamas have been increasing in recent weeks against the backdrop of a major Israeli operation against Hamas on the West Bank following the kidnapping and murder of three teenagers whose bodies were found last week.

Air strikes by Israel, both following the discovery of the bodies and in response to rocket fire, have escalated in recent days despite assessments by analysts in Gaza and Israel that neither Hamas nor Israel wants a prolonged or bloody conflict.

Despite the continuing rocket fire, Israel's prime minister, Binyanim Netanyahu, had shown a marked reluctance to be drawn into a military operation, offering Hamas "quiet for quiet" despite increasing political pressure from hardliners in his cabinet.

The Israeli foreign minister, Avigdor Lieberman, on Monday formally announced his party was leaving the Netanyahu coalition over the Gaza issue.

But despite this alleged government reluctance, the army has reportedly been ordered to prepare a significant expansion of its operation.

"We are entering into a long operation," a senior IDF source said. "We are only at the start. Patience is required. We are preparing further steps and a gradual expansion of our order of battle."

Nearly 300 rockets and mortars have been fired at Israel in recent weeks, including 80 projectiles on Monday alone, the Israeli army said. Israel has responded with dozens of air strikes in which eight Palestinian militants were killed on Monday, seven of them in a tunnel in an incident Israel claims occurred when Hamas members accidentally detonated their own explosives.

In a comment piece in Yediot Ahronot, Israeli journalist Alex Fishman suggested: "As far as it is concerned, Hamas' going ballistic over Israel will position it anew as a relevant player in the region and relieve it from its political and economic troubles. The leaders of Hamas are no longer discussing a ceasefire. They are trying to ride the wave of violence by Israeli Arabs and Palestinians in the West Bank with the purpose of garnering the support of the entire Palestinian public. And the solution for hitting Hamas without paying a price has not yet been invented, whether in the Israeli home front or in the international arena."

Israeli army spokesman Lt Col Peter Lerner said: "They [Hamas] chose the direction of escalation. 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."

Some analysts have suggested Hamas has been actively seeking to draw Israel into a ground conflict in Gaza.

As Operation Protective Edge launched, sirens sounded over large areas of Israel's south and air raid shelters were opened.

Among 50 sites the army said it targeted early on Tuesday were four houses belonging to militants, three militant compounds, 18 concealed rocket launchers, and other militant infrastructure sites. Most were targeted in air strikes, 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, including several suffering from shock. He said some of the injured Palestinians were treated and released.

Lerner said the army would gradually increase its attacks on Hamas in Gaza, and is recruiting additional reservists for a potential ground invasion.

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