Eyewitnesses said that the rocket accidentally hit the home of the al-Masri family in Beit Hanoun, injuring the pregnant mother, who suffered a miscarriage, and her two children.
According to the sources, the house and its surroundings suffered damage.
Earlier Thursday, another three rockets were fired at Israel , one of them landing on Palestinian soil and the other on Israeli territory.
The Sderot Municipality reported that the Color Red alert system identified the launching of a rocket at the western Negev at around 6:20 a.m. The landing site has yet to be located, but the rocket apparently landed in an open area outside the southern town.
Wednesday saw a record number of Qassam rockets launched in one day during the ceasefire which was declared last month. In total, seven rockets were fired on Wednesday and during the night, landing in the western Negev.
At least 40 rockets were fired from the Gaza Strip since the truce came into force.
A senior Israel Defense Forces official, talking to Ynet, warned that "Israel's policy toward the incessant Qassam rocket fire may be interpreted as a sign of weakness."
Ynet has learned that the IDF recently decided to remove artillery batteries that were placed around the Gaza Strip a few months after the disengagement, this following the errant fire on Beit Hanoun and the truce.
Defense Minister Amir Peretz said Wednesday evening that Israel would rethink its policy of restraint should Palestinian terror groups continue to breach the ceasefire. He noted that the continuous fire of Qassam rockets from Gaza into Israel is a blatant violation of the ceasefire.
"Israel has no intention to gamble with the safety of its citizens, there is a limit to our restraint," said Peretz.
Prime Minister Ehud Olmert also warned, during a meeting with his Norwegian counterpart Jens Stoltenberg , that Israel's restraint to ongoing violations of the ceasefire in the form of rocket attacks at southern Israeli cities would soon end.
"Over 40 rockets landed in Israel since the ceasefire and the last one was fired as we speak," Olmert said, referring to a Qassam rocket fired from the northern Gaza Strip at the western Negev by Islamic Jihad gunmen.
"Sometimes the difficulty has to do with others, who do not see the overall and complex picture we are experiencing in this part of the world. That is why we meet and speak and exchange views. I think this is the important thing, and not the words were sometimes hear and do not feel comfortable about," the prime minister added.
Halutz: Military need is not everything
IDF Chief of Staff Dan Halutz referred Thursday morning to Israel's restraint in an interview with Army Radio.
"When the prime minister decides to hold back, he is doing it out of a broader outlook than the direct military context of the need to respond to the Qassam fire. The military need alone is not everything," Halutz said, adding that "the right of self-defense is a right reserved to anyone."
Referring to the commission of inquiry into the Lebanon war , Halutz said, "My truth is my truth, and I don't plan to deviate from it and invent things. I will not make any special preparations because I do not believe I am coming to compete with Israel. I will not rent a lawyer in any case."
Meanwhile, Knesset Member David Tal (Kadima) attacked Prime Minister Olmert Thursday in light of his declarations that Qassam rocket fire would continue to be overlooked.
"How long will you overlook? Will it take 40 more Qassams or a rocket landing on a school for us to react?" Tal asked.
"Residents of the area live in constant fear and they must be provided the same peace and quiet as the rest of the countries residents," Tal concluded.
Ali Waked contributed to the report