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 signaled 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 ceasefire.
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.