"With the signing of this revitalised agreement, we should publicly acknowledge it is but one step on the road to peace, but one which lays the foundation for all that follows," he said.
His fears were echoed by Britain's Chris Trott, who spoke for the Troika bloc that also includes Norway and the United States and provides key funding to the peace process.
"We remain concerned about the parties' level of commitment to this agreement," he said, citing recent fighting in the northwestern city of Wau and the deaths of 13 aid workers in South Sudan this year alone.
The US last week said that 107 aid workers and 13 journalists had been killed since the war began.
"The Troika is committed to peace in South Sudan. But in order to be convinced of the parties' commitment, we will need to see a significant change in their approach," Trott said.
After decades of civil war, South Sudan voted to leave its northern neighbour Sudan in 2011, becoming the world's youngest country.
The split deprived Sudan of most of its oil reserves, and production was disrupted by the outbreak of war in South Sudan just two years after independence.
International frustration with the warring parties peaked in July, when the UN Security Council slapped an arms embargo and sanctions on two military officials to encourage South Sudan's leaders to turn away from the battleground and seek a diplomatic solution.
Efforts to revive the failed IGAD peace talks began in earnest in June this year with a face-to-face meeting between Kiir and Machar in the Sudanese capital Khartoum that led to the signing of a fresh ceasefire.
The peace deal comes as once-feuding governments in the wider, volatile Horn of Africa region move to settle long-held grudges.
In July, Eritrea and Ethiopia ended a two-decade-long border dispute, normalising relations and paving the way for Eritrea's expected return to IGAD.