Tom Fletcher spoke exclusively to The Associated Press after a two-day visit to Gaza, where hundreds of trucks carrying humanitarian aid have arrived each day since the ceasefire began on January 19.
"The threat of famine, I think, is largely averted," Fletcher said in Cairo.
"Those starvation levels are down from where they were before the ceasefire."
He spoke as concerns grow over whether the ceasefire can be extended and talks are meant to begin on its more difficult second phase.
The six-week first phase is halfway through.
As part of the agreement, Israel said it would allow 600 aid trucks into Gaza each day, a major increase after months of aid officials expressing frustration about delays and insecurity hampering both the entry and distribution of food, medicines and other badly needed items.
The UN humanitarian office has said more than 12,600 aid trucks have entered Gaza since the ceasefire took effect.
Fletcher urged both Hamas, which quickly reasserted its control of the territory in the hours after the ceasefire took effect and Israel to stick to the deal that has "saved so many lives."
"The conditions are still terrible, and people are still hungry," he said.
"If the ceasefire falls, if the ceasefire breaks, then very quickly those (famine-like) conditions will come back again."
The internationally recognised mortality threshold for famine is two or more deaths a day per 10,000 people.
For months before the current ceasefire, food security monitors, UN officials and others had been warning of possible famine in parts of devastated Gaza, especially the north, which had been largely isolated since the earliest weeks of the 16-month war.
Hundreds of thousands of Palestinians have been able to return to the north under the ceasefire.
"We can't ... sit by and just allow these people to starve to death," Cindy McCain, the American head of the UN World Food Program, told CBS in December.
The Biden administration repeatedly urged Israel to allow more aid deliveries and warned that failing to do so could trigger US restrictions on military support.
Fletcher said more food and medical supplies are crucially needed for the territory of more than two million people, most of them displaced, and he expressed concerns about disease outbreaks due to the lack of basic health supplies.
He also called for scaling up the delivery of tents and other shelters to those who have returned to their home areas, as winter continues.
"We must get tens of thousands of tents very rapidly in, so that people who are moving back, particularly moving back into the north, are able to take shelter from those conditions," he said.
Fletcher entered the Palestinian territory through the Erez crossing between Israel and northern Gaza, where he said he drove through "bombed-out, flattened and pulverised" areas.
He said he saw people trying to find where their homes had been and collecting the bodies of loved ones from the rubble.
He saw dogs looking for corpses in the rubble, too.
"It is a horror movie. It's a horror show," he said. "It breaks your heart again and again and again. You drive for miles and miles and miles, and this is all you see."
Fletcher acknowledged that some Palestinians have been angry at the international community over the war and its response.
"There was despair and anger. And I can understand the anger at the world that this has happened to them," he said.
"But there was also a sense of defiance as well. People were saying, 'We will go back to our homes. We will go back to the places that we have lived for generations, and we will rebuild.' "
