“This is a provocative aggression aimed at derailing the peace process,” Lam told Reuters.
But rebels launched coordinated attacks on the SPLA government army positions in four states, an SPLA spokesman said.
“The rebels wanted to gain more territory before a permanent ceasefire,” comes into effect, Lul Ruai Koang said in a statement to Reuters on Saturday.
On Wednesday, South Sudan’s president, Salva Kiir, signed a peace deal with rebels that included a ceasefire to start in 72 hours from the signing of the agreement.
But rebels led by Riek Machar, a former vice president, rejected parts of the deal, which comes ahead of a final settlement.
The country’s civil war began in late 2013, about two-and-a-half years after South Sudan gained independence from Sudan.
Previous peace deals have broken down and the war has uprooted a quarter of South Sudan’s population of 12 million, ruined the country’s agriculture and battered its economy.
Lam did not give details of casualties and said the fighting stopped late afternoon local time.
“We reserve the right to self-defence,” he said.