Gunfire erupted again in South Sudan's capital Juba on Tuesday, a day after President Salva Kiir said security forces had put down an attempted coup by supporters of his former deputy Riek Machar.

Constant gunfire and explosions in the early morning were followed by a few hours of relatively calm before sporadic gunfire resumed, witnesses said.

At least 26 people have been killed in the clashes in the capital, Makur Matur Kariom, Health Ministry undersecretary, told Reuters.

"These are the people we received at the hospital and who died actually at the hospital. Whoever died outside the hospital, we don't have their numbers," Kariom said from Juba Teaching Hospital.

Kiir imposed a dusk-to-dawn curfew on Monday to try to restore order. Residents say the security clampdown makes life that was already tough harder still.

"Food and water an issue for the population as they don't have fridges or city power so they buy food almost daily," said one aid worker in Juba, who asked not to be identified. "They haven't stocked up and are getting worried."