"We did not hear much from them that gave us hope that there will be rapid agreement," in peace talks, British ambassador Mark Lyall Grant told reporters, describing discussions with leaders as "disappointing."
Envoys met with President Salva Kiir in Juba, and held talks via a video link with rebel chief Riek Machar.
"Both said that they recognise that there was no military solution to the crisis, but the two positions remain far apart," Grant said.
The UN has said the food crisis is the "worst in the world", with aid workers warning of famine within weeks if conflict continues.
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Thousands of people have been killed and more than 1.5 million have fled civil war sparked by a power struggle between Kiir and his sacked deputy Machar, with battles between government troops, mutinous soldiers and ragtag militia forces divided by tribe.
"We underlined a very strong message that there will be consequences for those who undermine the peace process, that are not willing to put aside their personal agendas in the interest of the people," he added.
Stop-start peace talks in the Ethiopian capital Addis Ababa which began in January officially restarted again last week, but the delegates have made little if any progress.
Three ceasefire agreements have all been broken, while leaders missed a key deadline on Sunday to forge a unity government.
"We shall be discussing with them what the next steps are," Grant added.
The US and the European Union have already imposed penalties on three senior army commanders from the government and opposition, while IGAD nations have suggested they could follow suit if progress was not made.