World Food Program executive director Cindy McCain highlighted fears for the Northeast African country on the first anniversary of the eruption of fighting between rival generals leading the army and the paramilitary Rapid Support Forces.
McCain said on the sidelines of a Paris conference seeking funding for Sudan that the crisis had "already gone way too far."
Tens of thousands of people have been killed and 8.5 million more have been forced to flee their homes since the war started.
"So our goal now is to be able to get in, get access to the places we need. About 90 percent of the population can't be reached right now," she told AFP in an interview.
"And so when we start talking about is there famine, is there not famine, the truth is we don't know because we can't get in."
With organizers hoping some one billion dollars can be raised at the Paris conference, McCain said the international community until now "in my opinion, has not stepped up."
"They've left it to organizations like us, which they should. We're good at it. But there's no funding involved," said McCain, the widow of former U.S. presidential candidate and senator John McCain.
"So we can't feed unless governments and private entities are willing to fund what we do and make sure that famine does not begin in Sudan."
McCain said that despite the focus on other humanitarian crises, notably in Gaza, "we cannot forget places like Sudan."
"We have problems in South Sudan and Chad being able to function with all the refugees" who fled to the neighboring states, she said.
McCain lamented that U.N funding cuts were "devastating" for its food agency, making it hard to pre-position food at an arid time of year before the rainy season comes.
"And what I've been told is that unless we can get enough funding to do this, we're not going to be able to feed," she said, warning that operations in Chad, which is hosting Sudanese refugees, risked undergoing a "pause."
"It's very complicated. But it's also simple in some ways. We need food. We need to get food in," she said.
"This is a desperate situation. This could be and probably might be the largest humanitarian hunger crisis on the globe ever. And so that's why we've got to get in."
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