GAZA CITY, Gaza Strip (AP) — The heavy thud of tank shells, often just seconds apart, echoed across the Gaza Strip early Friday as thousands of Israeli soldiers launched a ground invasion, escalating a 10-day campaign of heavy air bombardments to try to destroy Hamas' rocket-firing abilities and the tunnels militants use to infiltrate Israel.

Flares lit up the night sky before dawn and the wail of ambulance sirens mixed with the Muslim call to prayer from mosque loudspeakers as thick smoke rose into the air from sites where shells and missiles struck.

"There is a tank shell every minute," said an official in the Gaza security operations room, who said all of the seaside strip's border areas were being shelled and that Hamas fighters were exchanging fire with Israeli troops near a northern Gaza town.

"There is also fire from the sea toward police checkpoints."

Israel launched the offensive late Thursday after becoming increasingly exasperated with unrelenting rocket fire from Gaza on its cities, especially following Hamas' rejection of an Egyptian cease-fire plan earlier in the week. Palestinian militants have fired more than 1,500 rockets at Israeli cities since fighting began.

However, a ground offensive could quickly lead to military and political entanglements for Israel, especially if more Palestinian civilians are killed.

More than 240 Palestinians have already died in the air campaign, including 14 children under age 12 killed over the past two days, according to Palestinian health officials. One Israeli has also died.

Israel accuses Hamas of firing from within populated neighborhoods and using civilians as "human shields."

Hamas struck a defiant tone. A spokesman, Fawzi Barhoum, said Israel "will pay dearly" for the assault. "Hamas is ready for a confrontation," he said.

The Israeli operation began around 10 p.m. Thursday, with what the military said was an open-ended assault to be carried out on several fronts.

"Large ground forces accompanied by massive air force support, naval forces and intelligence, are taking over targets in Gaza, operating against tunnels and terror activists and infrastructure," said chief military spokesman Brig. Gen. Motti Almoz.

He called on Gaza residents to evacuate targeted areas, warning the "military is operating there with very great force."

Gaza health officials said eight Palestinians were killed in the early stage of the ground operation, including a 3-month-old boy who died after a shell hit his family's Bedouin tent in southern Gaza. The body was evacuated on a donkey cart because ambulances couldn't reach the area due to heavy shelling, the officials said.

A resident of the northern town of Beit Lahiya, Jamal Abu Samra, said he was taking cover from the shelling by huddling on the ground floor of his home with his wife, six children and two dozen other relatives.

"We don't have power since the afternoon so we are listening to the (battery-operated) radio to hear the news," he said.

Israeli government spokesman Mark Regev said the operation was focused on the tunnels dug by Hamas under the Gaza-Israel border. Earlier Thursday, 13 heavily armed Hamas militants had tried to sneak into Israel through such a tunnel, but were stopped by an airstrike at the mouth of the tunnel.

"For Israel to send ground forces into Gaza is not a light decision. Ultimately we understand the risks involved both for our own soldiers and the dangers of escalation," he said. "But we felt this was necessary ... to deal with this strategic threat posed by those tunnels, which can allow terrorists to infiltrate into Israel and cause mass death."

Regev said "Hamas closed the door to a diplomatic solution."

"The people of Gaza are not our enemy. Our enemy is only those shooting rockets into Israel, trying to kill our people. In many ways the people of Gaza are also a victim of this terrible Hamas regime," he said.

Israeli officials have said the goal is to weaken Hamas militarily and have not addressed the possibility of driving the Islamic militants from power. However, Hamas has survived Israeli offensives in the past, including a major ground operation in January 2009 from which it emerged militarily weaker, but then recovered. Hamas has since assembled thousands of rockets and built a system of underground bunkers.

While the ultimate scale of Israel's ambition remained unclear, Prime Minister Benjamin Netanyahu had come under growing domestic pressure to ratchet up Israel's response to rocket fire that 10 days of airstrikes had failed to stem.

Israel has little stomach for the scale of casualties that a takeover of Gaza would likely entail, but Israeli public opinion appears to be nearly at a breaking point over the rockets.

Netanyahu may also have sensed he has a degree of international backing for action after Israel accepted an Egyptian cease-fire proposal Tuesday that was essentially a return to the status quo ante — and Hamas then rejected it. Similarly, Hamas ended a "humanitarian lull" of several hours Thursday by immediately resuming rocket fire.

However, the ground offensive brought swift criticism from U.N. Secretary-General Ban Ki-moon, who said he regretted that despite his repeated urgings and "those of many regional and world leaders together, an already dangerous conflict has now escalated even further."

Both Ban and the Obama administration took Israel to task for the mounting civilian death toll in Gaza.

"I urge Israel to do far more to stop civilian casualties," said Ban. "There can be no military solution to this conflict."

Jordan, the Arab representative on the U.N. Security Council, called for an emergency meeting of the council but no time was set. The body is already scheduled to hold an emergency meeting Friday morning on the shooting down of a Malaysian airliner in Ukraine.

Noting the deaths a day earlier of four boys who were killed on a Gaza beach by an Israeli strike, the State Department said the high civilian death toll in Gaza has been "heartbreaking."

Still, State Department spokeswoman Jen Psaki also criticized Hamas militants who continue to fire rockets and mortars into Israel, prolonging the latest round of violence.

Thousands of Israeli soldiers had massed on the border with Gaza in recent days, waiting for the order to go in.

Israel initially called up 48,000 reserve soldiers and later Thursday, the Cabinet authorized 18,000 more, the military said.

The ground operation followed a brief truce in which Israel held fire to allow Gazans to stock up on food and other necessities after being largely holed up at home since the conflict began last month.

Since July 8, Israeli strikes have hit more than 2,000 targets in Gaza and Hamas launched nearly 1,500 rockets at Israel, the Israeli military has said.

Israel last carried out a major ground offensive in Gaza in January 2009.

During that three-week campaign, some 1,400 Palestinians were killed, including hundreds of civilians. Thirteen Israelis also died.

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Deitch reported from Jerusalem. Associated Press writers Ibrahim Barzak in Gaza City and Yousur Alhlou in Jerusalem contributed to this report.