The United States has recalled its ambassador from South Sudan after the warring parties failed to form a unity government, the State Department said on Monday.

The US State Department said in a statement that Ambassador Thomas Hushek will return to Washington for consultations related to the recent failure of parties to form the transitional government by the extended deadline of November 12.

The Trump administration pointed out that Ambassador Hushek will meet with senior US government officials "as part of the re-evaluation of the US relationship with the Government of South Sudan given the latest developments".

Secretary of State Mike Pompeo tweeted that Washington would "work with the region to support efforts to achieve peace and a successful political transition in South Sudan."

South Sudan President Salva Kiir, opposition leader Riek Machar and a handful of other opposition groups signed a peace deal in September 2018. The rival leaders had been unable to create a unified army and determine the number of states since the deal was signed.

On November 7, the parties agreed to give themselves another 100 days beyond the November 12 deadline to form the transitional government.

The United States, the top humanitarian donor to South Sudan, said it was frustrated by rival leaders for failing to form the unity government as scheduled.