JUBA, South Sudan –  The U.N.’s top official for humanitarian assistance says aid workers in South Sudan are finding it hard to operate in the country because roadblocks are suddenly being erected in some areas.

Valerie Amos told reporters in the South Sudanese capital, Juba, on Monday that the situation is “worrying” as demands are being made of humanitarian workers before their convoys are allowed to proceed.

“The command and control mechanisms that should exist are breaking down,” she warned.

Some aid groups operating in South Sudan said in a statement Monday that more action is needed to avert a deepening crisis in the country. They also said that $600 million is needed to stock up on supplies before the end of the dry season in April.