Israel announced the start of a Gaza ground campaign on Thursday after 10 days of aerial and naval bombardments failed to stop persistent Palestinian rocket attacks, but it signalled the invasion would be limited in scope.
A statement from Prime Minister Benjamin Netanyahu's office said he had given orders to destroy tunnels that militants had dug to infiltrate Israel and carry out attacks.
An Israeli military spokesman said Israel was not out to try to topple the dominant Hamas Islamist group. Such a goal would likely entail a move into densely populated Gaza City, where urban warfare could prove costly to both sides.
Israel last mounted a large-scale invasion of the Gaza Strip during a three-week war in late 2008 and early 2009 that claimed 1,400 Palestinian and 13 Israeli lives.
No time frame was announced for the new operation, and the length and intensity of Israel's assaults could depend on the scale of civilian deaths - casualties likely to boost international pressure for a cease-fire.
Late on Thursday, Gaza residents and medical officials reported heavy shelling along the eastern border from the southern town of Rafah to the north of the strip. But there was no immediate sign that tanks, deployed for days near the border, were moving in.
Explosions echoed and flashes of orange lit the sky in the eastern Gaza Strip as Israeli gunboats fired shells and tracer bullets. Israeli artillery pounded the area and helicopters fired across the border, Reuters witnesses said.
Hamas spokesman Sami Abu Zuhri responded with defiance to Israel's invasion announcement.
"We warn Netanyahu of the dreadful consequences of such a foolish act," Abu Zuhri told Reuters.
Gaza health officials said 235 Palestinians, mostly civilians, had been killed since Israel began the air and sea offensive on July 8 in what it called a response to mounting rocket salvoes into its cities.
The warfare has been the worst between Israel and Palestinians in two years.
One Israeli has been killed in the current conflict. Many of the rockets, launched at Israel's south and the Tel Aviv metropolitan area, have been shot down by the Iron Dome anti-missile system, but the frequent fire has made a dash to shelter a daily routine for hundreds of thousands of Israelis.
A statement from the Israeli military said the operation will include "infantry, armored corps, engineer corps, artillery and intelligence combined with aerial and naval support".
It said another 18,000 reserve soldiers would be mobilized to join more than 30,000 already called up.
The ground operation began after Egyptian cease-fire mediation efforts hit a wall and warnings by Israel over the past few days to thousands of residents in Gaza's north and east to flee their homes for their own safety.
Israel briefly held its fire on Tuesday after Egypt, which is also Gaza's neighbor, announced a truce plan, but Hamas and other militant groups rejected the proposal, saying it had not addressed their demands.
Hamas wants Israel and Egypt, whose military-backed government is at odds with the Islamist movement, to lift border restrictions that have deepened economic hardship among Gaza's 1.8 million populace. Hamas is also suffering from a cash crunch, unable to pay its employees in Gaza for months.
Fighting resumed immediately after the end of a five-hour humanitarian truce on Thursday requested by the United Nations to allow Palestinians to stock up on food.
Before dawn, about a dozen Palestinian fighters tunneled under the border, emerging near an Israeli community. At least one was killed when Israeli aircraft bombed the group, the military said.
While tunnel-hunting incursions would be far short of a full-scale invasion, there is still the danger for Israel that risky and time-consuming missions could fall to Palestinian ambushes.
Hamas leaders have talked up their "tunnel campaign" against the Israeli enemy. One publicity video showed Palestinian fighters hauling rockets through a narrow passage to load onto a launcher that appears buried in an orchard. It is then fired remotely after its mechanized cover slides open.
On the diplomatic front, French Foreign Minister Laurent Fabius will visit Egypt, Jordan and Israel from Friday to Sunday to try to defuse the situation, and will discuss putting a European mission on the Gaza-Israel border, a diplomatic source said on Thursday.
The French diplomat said the mission could be similar to an EU mission launched in 2005 providing border help at the crossing in Rafah between Gaza and Egypt. That mission was suspended when Hamas was elected in 2007.
Fabius will meet Palestinian President Mahmoud Abbas and U.S. Secretary of State John Kerry separately in Cairo, Egypt's President Abdel Fattah al-Sisi and then the Jordanian foreign minister and head of the royal court. He will meet Israeli Prime Minister Benjamin Netanyahu in Tel Aviv on Sunday, the source said.
The conflict was largely triggered by the killing of three Israeli teens in the West Bank last month and the death on July 2 of a Palestinian youth in a suspected revenge murder. Israel indicted on Thursday three Israelis suspected of having killed the 16-year-old Palestinian in Jerusalem. A lawyer for a legal aid group representing the adult and two minors said they would enter a plea at a later date.
Additional reporting by Dan Williams and Maayan Lubell in Jerusalem; Writing by Jeffrey Heller and Maayan Lubell; Editing by Howard Goller