South Sudan's former vice president Riek Machar ordered his rebel troops to lay down their arms in line with a ceasefire to end a 20-month civil war, his spokesman said Saturday.

His rival, South Sudanese President Salva Kiir, had on Thursday ordered all government troops to cease fighting rebel forces as part of the agreement to end the conflict, which has left tens of thousands dead.

No clashes were reported between the two sides Saturday morning, according to Machar's spokesman and the regional eight-nation IGAD bloc which brokered the ceasefire deal along with the United Nations, the African Union, China, Britain, Norway and the United States.

The accord gave a 72-hour deadline for a permanent ceasefire, which comes into effect around sunset on Saturday.

The UN Security Council on Friday called for the ceasefire to begin immediately and threatened sanctions against those who undermine the accord.

Fighting erupted in December 2013 when Kiir accused Machar of planning a coup, unleashing a wave of killings that split the country along ethnic lines.

At least seven ceasefires have already been agreed and then shattered within days or even hours. Over two million people have fled their homes from a war marked by ethnic killings, gang rapes and child soldier recruitment. Some 200,000 terrified civilians are sheltering inside UN bases.

Under the peace deal, a "transitional government of national unity" will take office within three months.

Machar "gave a declaration of a permanent ceasefire to his troops last night," his spokesman Nyarji Roman told AFP on Saturday.

Kiir's spokesman, Ateny Wek, told AFP on Friday the president had ordered the entire army "to stop shooting and remain in their barracks where they are, but they can shoot in self-defence once attacked".

Late on Thursday, rebels accused the army of attacking their positions in the northern battleground state of Unity. The army did not respond, but has previously dismissed rebel claims as lies, or accused them of launching attacks.