An immediate monitored ceasefire must be the top priority of peace negotiations between the warring factions in South Sudan, Africa Minister Mark Simmonds said.
Both sides have agreed to begin talks tomorrow - a day later than anticipated - in the Ethiopian capital Addis Ababa in an effort to halt weeks of bloodshed.
Thousands have been killed and as many as 200,000 forced from their homes by fighting between troops loyal to president Salva Kiir and and those backing his sacked deputy Riek Machar.
Mr Simmonds said the situation remained "of grave concern" - with access required for humanitarian aid - and urged the parties to show flexibility.
"I am pleased that formal peace negotiations have been launched," he said.
"This is a constructive step and I urge both sides to remain flexible throughout these discussions.
"An immediate agreement to a monitored cessation of hostilities must be the priority. It's important that detainees are released quickly and with due process, to help ensure a successful dialogue.
"I welcome the pivotal role of the Intergovernmental Authority on Development and regional leaders, including Ethiopia in helping to bring the two sides together," he added - saying the UK The UK, US, Norway and other countries would "fully support this process.
"I hope these talks will help deliver the peace all of South Sudan's people wish for."
Earlier, South Sudan's foreign minister said the government was "serious" about securing an end to the fighting but warned that "compromise is for the other side".
Barnaba Marial Benjamin told the BBC that progress would require a recognition that the bloodshed was sparked by an "attempted coup" and that Mr Machar should pursue democratic means if he wanted to rule the country.
"Our side of the compromise is that we have accepted that there should be a negotiation.
"It is very important that we talk to each other in order to resolve this in the spirit of reconciliation and in a spirit that will help our people to stop their suffering and the fighting going on."