Juba - South Sudan's peace deal may collapse after rebel leader Riek Machar failed to return to the capital as planned, a ceasefire monitor and observers said on Wednesday.Machar was meant to arrive on Monday, and then Tuesday, to assume the vice presidency under his rival President Salva Kiir and form a transitional government. He failed to show up both days and it is not known when he will return.Machar's failure to come back to Juba puts the peace deal signed last year at risk, and the two sides should co-operate, said former Botswana President Festus Mogae, head of a commission monitoring the deal.The US accused Machar of making a "willful decision" not to return, threatening South Sudan with further conflict."We call upon the government to exercise maximum flexibility for the sake of peace, and on Machar to return to Juba," the US Embassy in Juba said in a statement on Wednesday.If Machar doesn't return to Juba very fast, "it can actually derail the implementation of the peace agreement", said Jacob Chol, a political science professor at the University of Juba.The rebels said Machar did not return because the government failed to clear a flight from Ethiopia for Machar's top general, Simon Gatwech, who is subject to an international travel ban under UN Security Council sanctions. The government said Gatwech was trying to bring at least 260 extra troops with him to Juba, contravening the peace deal.Besides non-formation of the transitional government, most key clauses of the peace deal haven't been implemented. Fighting continues, aid workers complain of restrictions on their access to suffering civilians, and the government has not complied with troop limits in Juba.Tens of thousands of people have been killed in South Sudan's civil war.