US Secretary of State John Kerry warned South Sudan's warring factions on Monday against walking away from ceasefire agreements and recently agreed peace talks, saying the implications of such a move would be "serious".
"Let me make clear: if there is a total refusal by one party or the other to engage into a legitimate promise which they agreed on... not only might sanctions be engaged but there are other serious implications and possible consequences," he said on a visit to Angola.
So far, US-backed diplomatic efforts to end a four-month long civil war in the world's youngest country have struggled to gain traction, amid allegations that both sides have committed war crimes.
South Sudanese troops and rebels are both reported to have committed crimes including mass killings, rape, attacks on hospitals and places of worship and recruiting child soldiers.
"There is accountability in the international community for atrocities, there are sanctions, there are possible... peacemaking forces, there are many number of possibilities," Kerry said.
"The parties need to recognise that they signed a cessation of hostilities agreements, both of them, and the international community is prepared to take steps to see that it is honoured, by putting additional troops."
The leader of South Sudan's rebels Riek Machar recently cast doubt on the prospect of imminent face-to-face talks with President Salva Kiir.
Last week Kerry, at the start of a four-nation Africa trip, urging both men to pull the country "back from the abyss".
But as Kerry spoke, the two sides were battling in the key oil town of Bentiu amid a major government offensive.