Secretary-General Ban Ki-moon called on the Syrian government Friday to immediately give humanitarian workers access to people who desperately need aid.
The U.N. chief spoke after government troops blocked a Red Cross convoy from delivering aid to an especially hard-hit neighborhood in the besieged city of Homs.
U.N. humanitarian chief Valerie Amos has been attempting, without success, to get permission from the Syrian government to visit.
Ban said President Bashar Assad's regime should let Amos into the country to assess the situation without delay. He said that ideally that should happen before the new special envoy to Syria, former U.N. Secretary-General Kofi Annan, meets with Arab League representatives in Cairo on Wednesday.
"The images which we have seen in Syria are atrocious," Ban told reporters several hours before reporting to the world body's membership on the situation there. "It's totally unacceptable, intolerable. How, as a human being can you bear this situation?
"This really troubles me. I'm deeply sad, seeing all that has happened," he added.
"The Syrian authorities must open, without any preconditions, to humanitarian communities," the U.N. chief said. "Why are they afraid of receiving the head of the U.N. humanitarian department?"
Ban told the General Assembly later Friday that he is "extremely disappointed" that Amos has not been able to travel to Syria "despite repeated assurances" by the Syrian government.
"I once again urge the authorities to allow her to visit, as soon as possible, so that humanitarian relief workers can reach the many thousands of people who desperately need assistance," he said.
The U.N. chief also said that Assad's government "has failed to deliver on its responsibility to protect its people. Civilian populations are under military assault in several cities.
"The disproportionate use of force by Syrian authorities has driven what had been largely peaceful opposition forces to resort to take up arms in some cases," Ban said. But he said the opposition's firepower is minimal compared with the government's heavy weapons.
The United Nations says more than 7,500 people have been killed in an 11-month crackdown by the government on protesters.
Syria's U.N. Ambassador Bashar Ja'afari took exception to Ban's comments, and insisted that his country had accepted a visit by Amos "in principle" and had not refused her access.
Ja'afari told the General Assembly that the resolution it passed on Syria two weeks ago was "unfair and "unilateral and subjective and completely unrelated to what is going on the ground in Syria."