Addis Ababa (AFP) - Peace talks between South Sudan's government and rebels resumed in Ethiopia on Tuesday, mediators said, urging both sides to return to a moribund ceasefire agreement.

The talks, mediated by regional bloc IGAD, are aimed at ending the three-month-old conflict and forging a lasting political solution between the government of President Salva Kiir and former vice President and now rebel leader Riek Machar.

"The IGAD special envoys are in consultation with the parties on the conduct of a political dialogue towards national reconciliation and healing," mediators said in a statement.

The violence, which has killed thousands and displaced nearly one million people, has persisted despite a ceasefire signed by both sides in January with clashes continuing in recent days in the oil-rich Upper Nile state.

IGAD condemned both sides for the "flagrant" violation of the ceasefire agreement, and said it reiterated its call "on the signatory parties to immediately adhere to the cessation of hostilities agreement," the statement said.

The fighting has taken on an ethnic dimension, with members of Kiir's Dinka tribe and Machar's Nuer tribe fighting each other.

Last week, the European Union and the United States threatened sanctions for both sides if they fail to progress with peace talks and stick to the ceasefire deal.

Observers have warned of a deepening humanitarian crisis in the region, with the number of refugees seeking safety in neighbouring countries growing daily.

The United Nations agency for refugees has said that $300 million (217 million euros) will be required to address the South Sudan refugee crisis by the end of the year.

Mediators in the Ethiopian capital said unfettered access was required to allow aid into the country, and urged both sides to comply.

"(We) call on the parties to ensure free access to humanitarian services to affected citizens and urge them to honour their commitment of opening up relief corridors across the country," they said.

Over 75,000 civilians are still crammed into UN peacekeeping bases in fear of revenge attacks, with conditions becoming increasingly squalid as weeks drag into months and heavy rains start.

• Politics & Government
• Unrest, Conflicts & War
• South Sudan
• Riek Machar
• IGAD