"The humanitarian situation in Gaza is a total disaster," Guterres said as he listed two reasons.
"First, a military campaign that has the highest level of killing and destruction that I remember in any other military campaign since I am Secretary General, anywhere in the world,” he said. “The second reason is because the level of humanitarian aid is totally out of proportion with the needs.”
His remarks came one day after Israeli Prime Minister Benjamin Netanyahu addressed a joint session of the US Congress, where he claimed that the war in Gaza has "one of the lowest ratios of combatants to non-combatant casualties in the history of urban warfare."
Turning to Netanyahu's claims about Gaza, Guterres said: "There was nothing said yesterday that is new. So, there is nothing that was said that deserves comments, and obviously, we absolutely must keep the two-state solution as the only possible long-term solution for peace in the region, independently of whatever is said by whoever, wherever."
Asked by Anadolu about his evaluations of Netanyahu's claims about the number of casualties in Gaza, Guterres said: "My answer is simple: to whom people believe in relation to that, I am very at ease in relation to this question."
Guterres said he has not reached out to Netanyahu while he is in the US to discuss attacks on UN convoys in Gaza.
"I have not reached out to the Prime Minister, but our people have been reaching out, both to Israeli authorities and also to other countries, exactly in order to make sure that this kind of regrettable incidents are not repeated," he added.
Netanyahu has faced a wave of protests since arriving Monday in Washington.
Thousands took to the streets to protest his address to Congress and his policies in Gaza. They are also demanding a cease-fire in the besieged enclave.
Nearly 39,200 Palestinians have since been killed, mostly women and children, and over 90,400 injured, according to local health authorities.
Over nine months into the Israeli onslaught, vast tracts of Gaza lie in ruins amid a crippling blockade of food, clean water and medicine.
Israel is accused of genocide at the International Court of Justice, which ordered Tel Aviv to immediately halt its military operation in the southern city of Rafah, where more than 1 million Palestinians had sought refuge from the war before it was invaded on May 6.