The United States has threatened sanctions against South Sudan if President Salva Kiir and rebel leader Riek Machar fail to form a unity government as promised.

South Sudan President Salva Kiir and rebel leader Riek Machar agreed in a rare meeting in September to form a government, but several deadlines have since passed without an implementation. 

Bryan Hunt, the U.S. State Department's office director for Sudan and South Sudan, warned that Western powers would not accept another delay in the deadline, which was already extended by six months.

"We're not prepared to continue to hear arguments for why more time must be given. We think it's past time, frankly, for the leadership to sit together and begin to find ways to move this country forward," he said.

"Our view is that if the government is not formed by November 12, we're going to need to re-evaluate the relationship between the United States and South Sudan," he told reporters at the US Institute of Peace.

Asked what measures could be taken, Hunt said options included sanctions targeting South Sudan's elite or restrictions on their travel to the United States.

Hunt disputed the claim that the U.S. was considering cutting off its $1 billion in annual assistance as it is largely humanitarian, supporting food and other basic needs among ordinary people.

"I don't think we should make them pawns or victims in a game of putting pressure on the South Sudanese government, however tempting that at times may be," he said.

The U.S. Ambassador to South Sudan, Thomas Hushek, also warned the U.S. remains on the lookout for spoilers of the peace process in South Sudan, adding that the U.S. executive order to sanction any official or individual blocking peace in South Sudan remains in effect.

"We don't want ill-gotten gains to be finding its way into our financial systems. If a country finds out that money that was stolen from the people here by corrupt officials, if it’s being invested in the U.S., sometimes sanctions can help a country get those ill-gotten gains back," he said.