South Sudan's army on Wednesday dismissed as "propaganda" rebel claims of a food blockade aimed at starving them into submission ahead of peace talks to end the 19-month civil war.
The United Nations and humanitarian agencies have also said aid is being restricted, including into areas already on the brink of famine.
But army spokesman Philip Aguer denied it had blockaded boats travelling on the Nile river, the main route for aid into the largely roadless northern areas, including areas under rebel control.
While boats were warned they risked coming under rebel attack, reports of a blockade were "mere propaganda not based on concrete facts," Aguer said Wednesday.
The rebels accuse the government of using aid restrictions as a tool to weaken them.
"The regime is starving these populations as another weapon of war to kill thousands more," rebel spokesman James Gadet said in a statement. "The pretext that our forces have been attacking barges which had been carrying relief assistance is false."
UN aid chief Stephen O'Brien warned on a visit to South Sudan last month that river barges carrying food and medical aid were being stopped.
O'Brien on Tuesday repeated the need for "rapid and unhindered access", recalling how he had urged President Salva Kiir to "ensure restrictions on Nile aid transport be lifted immediately."
South Sudan's civil war began in December 2013 when Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.
Regional mediators, backed by US President Barack Obama during his recent visit to Kenya and Ethiopia, have given South Sudan's rivals until August 17 to halt the civil war.
Talks were intially slated to start in Ethiopia's capital Addis Ababa on Wednesday, but neither side has given official confirmation of when they might turn up.
Obama on Tuesday also warned Kiir and Machar that if they fail to strike a deal the US will "move forward with a different plan, and recognize that those leaders are incapable of creating the peace that is required."
Accusing the government of "genocide" and intending to "starve the people to death", the rebels said government actions may "spoil the atmosphere of the expected peace negotiations in Addis Ababa."