"More than 80,000 Palestinians have now fled their homes because of the fighting around them ... there is a sense of overcrowding ... UN schools have taken in 35,000 refugees.
'Desperation and fear'
"There is real desperation and fear among the people," he said.
Mohyeldin also said that the so-called "humanitarian corridor" - the Israeli three-hour daily lull in fighting to allow food and medical supplies into Gaza - is "simply not producing a cessation of hostilities".
Shelling could still be heard in parts of Gaza City during the three-hour armistice, he reported.
Mads Gilbert, a surgeon with the Norwegian Aid Committee, told Al Jazeera: "This is a man-made situation that affects mainly the civilian population of Gaza who are without protection."
Thirteen Israelis have been killed in the conflict, including three civilians and 10 soldiers.
Alan Fisher, reporting for Al Jazeera from Israel close to the Gaza border, said around 15 rockets had been fired from the Strip into Israeli territory.
As the death toll continued to rise, diplomatic efforts to bring about a ceasefire appeared to make little progress.
Ban Ki-moon, the UN secretary general, arrived in Cairo earlier on Wednesday in a bid to kick-start ceasefire negotiations between Hamas - the Palestinian faction that controls the Gaza Strip - and Israel.
Ban met Hosni Mubarak, the Egyptian president, on arrival and is expected to hold talks with the leaders of Jordan, Syria, Lebanon and Turkey.
The UN chief has not said whether he will have direct contact with Hamas leaders.
Ban has repeatedly called for both sides to immediately end hostilities, so far to no avail.
Robert Fisk, a journalist and Middle East expert, said neither the current Gaza war nor the broader 60-year regional conflict would end without resolving the Palestinian issue.
"Unless we deal with... [Palestinian refugees], there will not be an end to this war"
Robert Fisk, journalist and Middle East expert
"Why are they [Palestinians] dispossessed? Why are settlements - colonies for Jews and Jews only - being built on Arab land illegally? And still it continues," he told Al Jazeera.
"Unless we deal with this [Palestinian refugees], there will not be an end to this war. There might be a ceasefire in Gaza, a ceasefire in the West Bank, but there will not be an end to the war. That is the problem."
Earlier this week, the United Nations Security Council agreed a binding resolution demanding an immediate ceasefire in the Strip.
However, both Israel and Hamas have ignored it and continued fighting.
Fisk said that Israel will be able to flout the UN ceasefire demand as long as the US - the only country to abstain from the 15-member security council vote on the resolution - continues to back Israel.
"It's quite clear from Hillary Clinton [incoming US secretary of state] most recent comments that it [the US backing of Israel] will continue under Barack Obama.
"I see no change, I see no hope at all in the future," Fisk said.