Days of heavy fighting in the world's youngest country last week left hundreds of people dead.
It followed clashes between troops loyal to President Salva Kiir and Vice-President Riek Machar.
The violence mirrored events of December 2013, when a two-year civil war began.
A ceasefire declared last Tuesday appears to be holding.
African leaders are meeting in the Rwandan capital, Kigali, where they will discuss among other things the crisis in South Sudan.
Chad's President and current chair of the AU, Idriss Deby, says the upsurge in fighting is unacceptable.
"The recent violence which occurred in South Sudan and which started before the beginning of this summit, is a reminder to us of the need to adapt our organisation to the reality of the time we live in. You can see our incapacity to face many crises on this continent. There is fighting in South Sudan where 300 people died and thousands fled their homes: this is happening while there is a big international effort to stabilise this country. This is unacceptable and this can spread everywhere on this continent."
United Nations Secretary General Ban Ki-Moon is also attending the AU summit.
Mr Ban has renewed his call for an immediate arms embargo on the young country.
"I proposed three things: first, imposing an arms embargo immediately. The second, sanction those people who are responsible, who have ben inciting, provoking this situation and thirdly, our UN peacekeeping mission, UNMISS, should have capacity fortifying our mission by increasing the capacity of United Nations missions including allowing (them) to use attack helicopters and any other means."
However South Sudan's Ambassador to the AU, James Pitia Morgan, has told the BBC his country will not allow any additional foreign troops.
"We don't think that South Sudan needs a lot of forces. The forces are enough, that is what the president was saying. These forces are enough here."
Meanwhile, the UN's food agency, the Food and Agriculture Organisation, is warning that millions of people facing hunger in South Sudan could be driven to the brink of catastrophe if violence continues.
It says more than 4.8 million people are severely food insecure and malnutrition rates are high.
