US officials estimate that as many as 1.25 million people live in areas humanitarian workers cannot access.
Between March 27 and May 5, the government, the World Food Programme (WFP) and other partners provided food aid to more than 1.2 million people in 32 districts.
Humanitarian workers say famine could have already arrived there and they would have no way of knowing.
"We are seeing a trend towards more restricted humanitarian access in Tigray," a spokesperson told CNN.
CNN reported Wednesday that Eritrean troops, in coordination with Ethiopian forces, had once more closed off key aid routes in central Tigray.
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