“I was prepared for nightmare, but it is worse, much worse,” Mr Egeland told the BBC on Sunday.
“People want to take your hand… saying ‘we are starving, we are dying here’.
“I think there is famine in the north,” he said, adding that there had been no aid for 300,000 people living in ruins, with Israel not allowing any through.
After Thursday’s aid convoy incident, the US carried out its first airdrop of humanitarian aid for Gaza, with more than 30,000 meals parachuted in by three military planes on Saturday.
Elsewhere, Israel said on Sunday it carried out an intensive wave of air strikes in the southern Gaza city of Khan Younis. The number of casualties is not known.
At least 11 people were killed in an Israeli air strike at a camp for displaced people in Rafah in southern Gaza on Saturday, according to Hamas.
World Health Organization chief Tedros Adhanom Ghebreyesus called the attack “outrageous”.
The Israeli army said it had carried out a “precision strike” against Islamic Jihad militants in the area.