Overnight, two Israelis were killed by Hamas rockets, bringing the total number of Israelis killed to four since Saturday.
The Bush administration spoke out Monday on the Israeli assault of Hamas targets throughout Gaza, condemning Hamas for the escalation of violence.
"In order for the violence to stop, Hamas must stop firing rockets into Israel and agree to respect a sustainable and durable cease-fire," White House spokesman Gordon Johndroe said Monday. "That is the objective to which all parties need to be working. That is what the United States is working towards."
Israeli Foreign Minister Tzipi Livni told ABC News that "Hamas represents an extreme Islamic ideology. They are not fighting for Palestinian rights. They are to deny us our right to exist."
In Gaza, conditions have worsened for its 1.5 million residents. The territory's nine hospitals report they are overwhelmed by the wounded. Palestinian residents are digging through the rubble to find and free survivors. On Gaza's southern border, Egypt shut its crossing after thousands of Gazans tried to escape the fighting and breached the border wall.
The human cost of the Israeli bombing of Gaza is enormous. Among the hundreds killed are five sisters, age 4 to 17. They were crushed to death Monday when a wall collapsed in their home after an Israeli airstrike.
One of the surviving sisters spoke to ABC News: "We were sleeping. We woke up and we were buried in the rubble. My 4-year-old sister sleeping next to me died."
Two more young sisters, aged 4 and 11, were killed today in another Israeli strike in northern Gaza.
At Gaza's biggest hospital, Shifa, the injured keep pouring in with each new airstrike. Since Saturday, the hospital has treated more than 200 people with traumatic injuries, even though its intensive care unit is designed to serve just 25.
Medical supplies in the hard-hit Gaza enclave are few and food stocks are rapidly being depleted. Shifa's Dr. Kamal Abada told ABC News that they are running especially low on essentials like bandages, sutures and critical medicine.
Saturday night the hospital itself was hit. Shifa workers collected the injured from their own parking lot. Since the hospital's glass windows were shattered in the attacks, the nights are so cold that attendants cover the windows with plastic sheeting.
Dr. Rawea Awad is now a patient, after she suffered a massive head injury on her way to the hospital. Abada admitted that he and his colleagues are afraid to come to work, but he said they are more afraid to stay home.
Israel has allowed some supplies in, offering hope for medical aid and generators to reach the hospital. The Israeli Defense Force said today it opened the gates for 110 trucks of medicine, food, cooking oil, sugar and flour.
In addition, Egypt opened its Rafah crossing to allow 30 to 35 trucks filled with tents, blankets and medical supplies for those who had lost their homes, according to the Red Crescent charity. Most of the aid that came through Rafah was provided by Qatar, Saudi Arabia, Libya, and Egypt.
But if there is an Israeli ground offensive the level of human suffering is expected to grow much worse with both sides having to engage in fierce urban fighting.