Palestinian militant group Hamas has said there will be no let-up in its attempts to launch missiles from Gaza at targets in neighbouring Israel.
Palestinian sources say 100 tanks have moved into northern Gaza
The statement came as a major Israeli operation in Gaza to halt the rocket attacks continues.
Seven Palestinians have been killed in the latest violence, bringing the total killed since Wednesday to at least 40.
Late on Saturday, an explosion hit a car in Gaza City, killing two well-known middle-ranking Hamas militants.
Hamas sources identified the men as Mehdi Mushtaha and Khaled Amreet.
Israel has not confirmed the attack - which is similar to many the army has carried out before to target Palestinian militants.
The Israeli operation in Gaza began after two children were killed by a rocket in the Israeli town of Sderot on Wednesday.
The Palestinian leadership described Israel's actions, as its forces advanced into the densely crowded Jabaliya refugee camp, as a war crime, and has called for international intervention and humanitarian assistance for the people of Gaza.
It has also declared a state of emergency.
Palestinian President Yasser Arafat said: "I appeal to the world to ... stop these inhumane and
His Prime Minister, Ahmed Qurei, has cancelled planned visits to Russia and Turkey.
The US has called on Israel to limit its latest offensive, and France and Russia have voiced deep concern.
Israel says that no nation would tolerate the random bombardment of its civilian areas.
Prime Minister Ariel Sharon has ordered his troops to take all means necessary to stop the attacks.
And the Israelis have released pictures from surveillance aircraft apparently showing Palestinians loading rockets into a United Nations-marked vehicle.
The UN is investigating whether one of its vehicles was really used.
Hamas made its statement vowing to continue the rocket attacks in the Jabaliya camp.
The BBC's Alan Johnston reports that four heavily-armed Hamas men wearing masks strode into a room in a mosque in the middle of the camp.
Palestinians have fought running battles with Israeli troops
He says they stood behind a table set out with grenades and rocket launchers, and delivered a message that was as uncompromising as ever.
A spokesman said Hamas would continue to launch rockets at targets like Sderot, which lies inside Israel just beyond Gaza's perimeter, as Hamas regarded anyone living there as a target.
He said they were all part of the Israeli state that had subjected the Palestinians to occupation.
Hamas wants an end not only to the occupation of Gaza and the West Bank. It says that Israel itself is on land that belongs to the Palestinians.
The Israeli military operation marks the first time for more than two years that Israeli tanks have entered the Jabaliya camp, which is home to more than 100,000 people and one of the most crowded places in the world.
Israeli attacks have not stopped rockets from reaching Israeli towns
At least 27 Palestinians and three Israelis died in fighting in Gaza on Thursday - one of the bloodiest days since the Palestinian uprising (intifada) began in 2000. Many civilians were among the dead and injured.
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.