The US has called on Israel to exercise restraint in its latest offensive against Palestinian militants in northern Gaza.
Palestinian sources say 100 tanks have moved into northern Gaza
The state department said Israel should use only "proportional force" and urged it to avoid civilian casualties.
More than 40 Palestinians and three Israelis have died since the army's operation began on Wednesday.
On Friday, Israel sent up to 100 tanks into northern Gaza, after the cabinet ordered a major military operation.
On Saturday, six Palestinians were reported killed in two separate incidents.
The Israeli military said its soldiers shot and killed four
armed militants as they tried to infiltrate an
Israeli farming village along the Gaza border.
Another two Palestinians died and four more were injured in an air strike on the Jabaliya refugee camp.
Officials said the two were members of the Islamic Jihad militant group.
The attack came hours after Israeli special forces reportedly killed one Palestinian north of the camp when they stopped a group of seven men from carrying out an attack on the Erez border crossing between Gaza and Israel.
The open-ended ground operation - codenamed Days of Penitence - aims to stop Palestinian rocket attacks on Israeli border towns, officials say.
The United States said Israel - Washington's closest Middle East ally - had a right to defend itself, but warned against using excessive force.
"All care should be taken to protect and respect civilian life and property," said deputy State Department spokesman Adam Ereli.
Palestinian sources say scores of tanks have moved into Jabaliya and the towns of Beit Hanoun and Beit Lahiya.
It is the first time for more than two years that Israeli tanks have entered the Jabaliya camp, home to more than 100,000 people and one of the most crowded places in the world.
The BBC's Alan Johnston in Gaza says the tanks have thrust into the centre of a stronghold of groups like the Hamas organisation.
At least seven Palestinians were killed in the camp on Friday, according to Palestinian sources, but, our correspondent says, the exchanges were nothing like as intense as those on Thursday.
Also on Friday:
An Israeli helicopter fires at least two missiles into Gaza City, which the Israeli army said targeted a suspected weapons storage facility
A Palestinian rocket falls in the
Israeli border town of Sderot, but no-one is injured, medics and witnesses say.
At least 27 Palestinians and three Israelis died in fighting in Gaza on Thursday - one of the bloodiest days since the intifada began in 2000. Many civilians were among the dead and injured.
A day earlier, a Qassam rocket fired from northern Gaza killed two children in Sderot.
'Days of Penitence'
Zalman Shoval, a senior adviser to Israeli Prime Minister Ariel Sharon, told the BBC on Friday: "No government can allow a situation to develop where terrorists launch missiles and mortar shells at civilian populations."
But Palestinian cabinet minister Saeb Erekat said Israel was using "disproportionate force" in the operation.
The Palestinian Authority has called for an urgent meeting of the UN Security Council to debate "barbaric massacres" by Israeli troops in Gaza.
The Gaza Strip has been occupied by Israel since it captured the territory in the 1967 war.
Palestinian rocket attacks are complicating Mr Sharon's plans to end Israel's occupation of Gaza, observers say.
Militants are keen to portray any eventual Israeli withdrawal as a retreat under fire, and many expect the violence to escalate.