The United States, Britain and Norway have called on parties in South Sudan's conflict to stop violating a cease-fire signed last month, their heads of mission in the capital Juba said on Tuesday.
The deal aimed to end a four-year war between the government of President Salva Kiir and rebels in which tens of thousands of people have been killed.
But since the signing of the deal in the Ethiopian capital Addis Ababa, there have been several reported violations for which both sides have been blamed.
The United States, Britain and Norway form a group that supported the 2005 accord leading to the independence of South Sudan from Sudan. They have threatened to impose individual or group sanctions for those violating the cease-fire.
"We call on all signatories, and the field commanders who answer to them, to immediately end all military operations," the three Western countries said in a statement.
They added that field commanders and their political bosses would be held accountable for violating the cease-fire and impeding humanitarian assistance.
The ceasefire is also designed to allow humanitarian groups access to civilians caught in the fighting and revive a 2015 peace deal that collapsed in 2016 after heavy fighting erupted in Juba.
On Tuesday, a spokesman for a faction now allied with the government said rebels had attacked their positions on December 31.
He said one of the attackers was killed and two were captured alive in the incident.
The cease-fire was meant to be followed by talks on a revised power-sharing arrangement leading to a new date for polls.
The war in South Sudan, the world's youngest country, was triggered by Kiir's decision in 2013 to sack his deputy, Riek Machar. It has been fought largely along ethnic lines between forces loyal to Kiir, an ethnic Dinka, and Machar, who is Nuer.
The war has forced a third of the 12 million-strong population to flee their homes.
On Monday, the U.N. refugee agency (UNHCR) said it was investigating an allegation by one of the rebel groups that Kenyan security forces unlawfully deported one of their senior members from Kakuma refugee camp over the weekend.
Rebel spokesman Lam Paul Gabriel said the deportation from Kenya was a violation of the cease-fire.
Spokesmen for the Kenya police and the foreign affairs ministry told Reuters on Tuesday they were not aware of the incident.
South Sudan's presidential spokesman Ateny Wek Ateny told Reuters the Juba government had no link to any missing citizens in Kenya.