"I think we're in slightly more difficult territory at the moment," UN humanitarian chief Martin Griffiths told reporters.
He insisted though that the arguments for continuing were "conclusive and persuasive," stressing that "the Global South ... needs that operation to continue."
Ukraine is one of the world's top cereal producers.
After Russia launched its invasion nearly a year ago, 20 million tons of grain were blocked in its ports until a safe passage deal was agreed.
The hard-won Black Sea Grain Initiative was brokered by the UN and Turkey last July. It was renewed a first time in November and is up for renewal again on March 18.
Nearly 20 million tons of cereal have been exported since then, said Griffiths, insisting: "We don't need it stopped in the middle of March."
"I hope and I believe, actually, that it will be extended. That is because it is an obvious case for international humanitarian security."
Ukraine appealed on Wednesday to the United Nations and Turkey to press Russia to immediately stop hindering Ukrainian grain shipments that supply millions of people and not to use the food as a weapon.
Two top Ukrainian officials said in a joint statement that "Ukraine is deeply concerned about the destructive actions of Russia", which result in the delay of the work of the grain corridor and "obstructing the Black Sea Grain Initiative in general"