Aid workers in South Sudan are finding it hard to operate in the country because roadblocks are suddenly being erected in some areas, the U.N.'s top official for humanitarian assistance said on Monday.

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

"Roadblocks are just being suddenly established and demands being made of humanitarian workers that they pay to allow the convoys to go through," she said. "This is reflection of the fact that the command and control mechanisms that should exist are breaking down."

Many aid workers risk being kidnapped and have seen their supplies and equipment looted, she said.

South Sudan has been plagued by violence since December 2013 when fighting erupted between pro-government forces and soldiers loyal to former Vice President Riek Machar. Despite multiple peace agreements being signed between the warring factions, the threat of violence remains across the country.

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.