Thousands of Palestinians are fleeing northern parts of Gaza after Israel warned it was targeting the area in its campaign to stop the continuing rocket attacks from Gaza militants.
The UN says 17,000 people have sought refuge in its facilities.
On Sunday, Israeli forces raided a suspected rocket launch site in Gaza in their first reported ground incursion.
At least 172 Palestinians have been killed since Israel's offensive began seven days ago, officials in Gaza say.
The dead are said to include 18 members of one family who died in an Israeli missile strike on Saturday evening.
A spokesman for the Israeli Defense Forces (IDF), Lieutenant-Colonel Peter Lerner, disputed the casualty figures, saying they were not objective.
He told the BBC that Israel had aborted certain attacks for fear of killing civilians.
Israel says it is targeting Hamas militants and "terror sites", including the homes of senior operatives.
However, the UN has estimated that 80% of the people killed in Gaza have been civilians.
The UN Security Council has called for a ceasefire and peace talks.
In the latest developments:
- An aerial drone from Gaza infiltrated Israel on Monday morning, and was shot down by a Patriot missile above Ashdod
- Israeli air strikes early on Monday hit three training facilities of Hamas's military wing and buildings in Gaza city, causing injuries, AFP reported
- The Israeli military said Gaza militants had fired 130 rockets into Israel on Sunday
- A rocket fired from Gaza hit electricity infrastructure in Israel that supplied power to Gaza, cutting power to about 70,000 people, the Israeli military said
'Nowhere to go'
The military confirmed it had dropped leaflets over the city of Beit Lahiya on Sunday morning telling civilians to seek shelter.
"We do not wish to harm civilians in Gaza, but these civilians must know that remaining in close proximity to Hamas terrorists and infrastructures is extremely unsafe," the IDF said.
UN Relief and Works Agency spokesman Chris Gunness said in a tweet that the agency had doubled its spaces for displaced people from 10,000 to 20,000.
Meanwhile, about 800 Palestinians holding dual citizenship began leaving Gaza via Israel's Erez Crossing.
However, Gaza resident Sawla el Tibi told the BBC it was too dangerous to leave her home. "[There is] no safety at all to walk on the street... all of Gaza Strip is burning right now," she said.
The Palestinian Authority's envoy in the UK, Manuel Hassassian, told BBC News there was nowhere for Gaza residents to hide.
"There are no shelters, no bunkers, no place to go, except their homes," he said. "If they leave their homes, they will be hit on the street."
Early on Sunday, Israeli air strikes destroyed most of the security headquarters and police stations run by Hamas Islamist militants.
Israel has been building up its troops along the border with northern Gaza, fuelling speculation of a possible ground invasion.
Defending Israel's actions, Prime Minister Benjamin Netanyahu told US broadcaster CBS: "We're sorry for any accidental civilian deaths but it's the Hamas that bears complete responsibility for such civilian casualties."
The IDF says it has so far struck some 1,320 "terror" sites across Gaza, while Hamas has launched more than 800 rockets at Israel.
At least three Israelis have been seriously injured since the violence erupted, but no Israelis have been killed by the attacks.
A Palestinian health ministry spokesman has said 1,260 people have been injured in Gaza.
France on Sunday again condemned the Hamas rocket attacks, but also called on Israel to "show restraint" in its Gaza campaign and avoid civilian casualties.
Germany is sending Foreign Minister Frank-Walter Steinmeier to Israel on Monday for talks with Israelis and Palestinians to help negotiate an end to the violence.
Rocket fire and air strikes increased after the abduction and killing of three Israeli teenagers in June and the suspected revenge killing of a Palestinian teenager in Jerusalem.
Israel and militants in Gaza fought an eight-day war in November 2012, which ended with a truce.
Israel's Iron Dome missile shield
- Enemy fires missile or artillery shell
- Projectile tracked by radar, data relayed to battle-management-and-control unit
- Data analysed and target co-ordinates sent to the missile firing unit
- Missile is fired at enemy projectile
Correction 26 August: The UN figure for the number of estimated civilian fatalities has been amended after an earlier version cited 77% based on an out-of-date report.