The negotiations have made scant headway beyond a ceasefire deal signed in January but which later collapsed. Some Western diplomats question the commitment of either side to end the conflict, with each blaming the other for truce violations.
Negotiations mediated by the east African grouping Igad are due to resume in Addis Ababa on Thursday after a two-week break, but diplomats said it was unclear if both sides would be there.
"If the government and any other actor tries to undermine the peace process and rebuff the Igad heads of state, they will face consequences," said a joint statement by the United States, Britain, Norway and the European Union read by Donald Booth, the US envoy to Sudan and South Sudan.
Booth and Alexander Rondos, the EU special envoy to the Horn of Africa, said Washington and Brussels were both considering "targeted measures" against individuals, but gave no details.
Reuters revealed in January that Washington was weighing targeted sanctions against South Sudan due to the failure of leaders in the world's youngest nation to act to end a crisis that has thrust it to the brink of civil war.
Major sticking points are rebel demands that four remaining political detainees jailed on suspicion of plotting a coup with former vice president Riek Machar be freed, and the withdrawal of Ugandan troops.
Ugandan forces entered South Sudan on the invitation of the government to help protect the airport, presidential palace and other installations in capital but have been accused by rebels of fighting alongside government forces in various flashpoints.
Booth said the government had announced on state media that it would boycott the talks if seven other politicians - previously jailed alongside the four - were involved.
South Sudan's supporters and regional neighbours fear unrest in the world's newest country could spill beyond its borders and destabilise a volatile region which has in recent years enjoyed strong economic growth.
South Sudan, which gained independence from northern neighbour Sudan in 2011, narrowly avoided civil war when troops loyal to sacked Vice President Riek Machar rose up against the government of President Salva Kiir.
Thousands have been killed in the ensuing violence, which has often pitted factions from Kiir's Dinka group and the Nuer ethnic group of Machar against each other.
"Patience is beginning to run a little thin with the parties to this conflict who think that they can toy with their own agreements and with their neighbours and eventually at the cost day by day of lives of their own citizens," Rondos said.