There is growing international concern about fighting between rival armed groups in South Sudan.
Thousands of people have fled over the border to the Democratic Republic of Congo in recent days to escape the violence.
Norway, the UK and the US – known as the Troika – say they are particularly disturbed by the situation around Yei.
They have called on the government of South Sudan, the forces loyal to the former Vice-President Riek Machar and another rebel group, the National Salvation Front, to end the violence immediately and to respect last year’s peace agreement.
The Troika also called on regional leaders to respect the UN arms embargo.