The UN Security Council warned Thursday of "cost and consequences" for any side in South Sudan that blocks a new regional peace initiative set to open next week in Ethiopia.

In a unanimous statement, the council said a high-level forum organized by the IGAD East African group "is a unique window of opportunity, but equally a last chance for the parties to achieve sustainable peace and stability in South Sudan."

The so-called high-level revitalization forum will open in Addis Ababa on Monday, although it remains unclear who will attend.

The council "strongly urges all parties to engage constructively in the process" and underlines that "there must be cost and consequences for those who undermine" the forum, said the statement.

British Ambassador Matthew Rycroft said there were "clear expectations" that the sides will take part and that failure to meet those could result in an arms embargo or targeted sanctions.

The United States failed last year to garner enough support for an arms embargo, but diplomats say the measure could once again be examined by the council in January with stronger backing if the peace effort fails.

South Sudan this month will enter into its fifth year of a devastating war that has killed tens of thousands of people and driven nearly four million South Sudanese from their homes.

Last month, the United States, South Sudan's biggest aid provider and a key supporter of its 2011 independence, threatened to take unspecified measures against President Salva Kiir's government.

The threat followed a visit by US Ambassador Nikki Haley to the capital Juba in October, where she held talks with Kiir -- becoming the highest level US administration official to visit South Sudan.

Kiir wants elections to be held next year to cement his hold on power, but the council said in the statement that "conditions for credible elections do not presently exist."

The council also renewed the mandate of its peacekeeping mission in South Sudan for three months as it awaits the recommendations of a review of the force.