"There is no coming today," Minister of Information Michael Makuei said, adding that the government will issue flight clearance for Machar to arrive by plane from Ethiopia only after international monitors have verified the number of weapons carried by the rebels accompanying him.
The rebels, who were at an airport in Ethiopia, said they were ready to fly but needed permission to do so.
UN chief Ban Ki-moon has urged Machar to return to Juba "without delay", while the US, Britain and Norway - key international backers of peace efforts - demanded he return by today.
Machar was due to return to the capital Juba on April 18 to take up the post of first vice president alongside arch-rival President Salva Kiir.
His failure to arrive has thrown an August 2015 peace agreement to end over two years of intense civil war into jeopardy.
Machar can bring with him 195 men, carrying AK-47 assault rifles as well as 20 machine guns and 20 rocket-propelled grenades.
Rebel spokesman James Gadet however said the weapons had already been checked by Ethiopian officials, and could also be verified upon their arrival in Juba.
"The forces are ready," Gadet said.
