"Two consecutive shells just landed in the busy car park 15 metres from the entrance to the emergency room," Alberto Arce of the International Solidarity Movement said in a statement.
"The entrance of the emergency room was damaged. At the time of the shelling ambulances were bringing in the wounded that keep pouring in."
On Sunday, an Israeli raid killed at least four paramedics as they tried to reach wounded Palestinians. Ambulances have also been hit in the attacks, Palestinian sources said.
Israeli government officials say they are not targeting civilians, but only seeking to halt rocket fire from the Palestinian Hamas movement governing Gaza.
There are also fears that the humanitarian situation will further deteriorate as the strip, home to 1.5 million people, is suffering from acute shortages of fuel, food and medical supplies.
Iyad Nasr of the Red Cross in Gaza City said that the military operation has worsened the hardships created by the Israeli blockade over the last 18 months.
"The size of the operations and the size of the misery we are seeing here on the ground is just overwhelming," he said.
"We are trying our best to support the infrastructure that has been depleted ... and prevent the total collapse of the medical systems.
Nasr also said that aid workers and emergency medical personnel were finding it increasingly difficult to move around the territory after the Israeli military effectively split it in two.
"The ICRC has to contact the Israeli authorities for each single wounded to be evacuated with an ambulance," he said.
Al Jazeera's Ayman Mohyeldin, reporting from Gaza City, said that many other Gazans have fled their homes taking refuge in schools converted into temporary shelters by UN agencies.
"The United Nations says 13,000 people, over 2,000 families, have now been internally displaced because of the fighting, and that is just in the north of the strip," he said.
'People are suffering'
Tzipi Livni, the Israeli foreign minister, has said that there is no crisis and that aid is getting through, but Christopher Gunness, the UN Relief and Works Agency (Unrwa) spokesman, said her denials were absurd.
"The organisation for which I work - Unrwa - has approximately 9,000 to 10,000 workers on the ground. They are speaking with the ordinary civilians in Gaza... People are suffering," he said.
|Thousands of Palestinians have fled their homes and taken refuge in schools [EPA]
"A quarter of all those being killed now are civilians. So when I hear people say we're doing our best to avoid civilian casualties that rings very hollow indeed."
About 250,000 people in the northern part of Gaza are also reported to be without electricity. The main power plant has been shut down for lack of fuel due to Israel's blockade.
The British-based Save The Children charity on Monday warned that newborn babies in the Gaza strip were at risk of hypothermia because of the power cuts and freezing winter temperatures.
"We need to deliver more food and blankets to ensure that children do not die of hunger and cold," Dominic Nutt, a spokesman for the group, said.
"People also must be able to move freely and safely so they can provide for their families when food does become available."
Despite the crisis in Gaza, Mahmoud al-Zahar, a senior Hamas official, said the group was heading for "victory" against the Israeli military.
He said that Hamas's armed wing, the Izz-e-din al-Qassam Brigades, had "given the most beautiful performances during its confrontation with the army that the world thought invincible".
Palestinian factions have continued to launch rockets into southern Israel, despite more than a week of aerial bombardment by Israel and the ground offensive.
One Israeli soldier has been confirmed killed in the Gaza assault so far, with at least 49 others wounded. Four Israelis have also been killed by Palestinian rockets.