Juba - South Sudan on Wednesday rejected a US proposal for the UN Security Council to send 4 000 additional troops to the East African country to restore calm, saying it "seriously undermines" its sovereignty and threatens a return to colonialism.Government spokesperson Michael Makuei said the proposal gives the UN the ability to govern. The proposal also calls for a vote on an arms embargo on South Sudan if UN Secretary-General Ban Ki-moon reports within a month that authorities have blocked the regional force.The Security Council could vote on Friday on the proposal, which comes after a former US special envoy suggested last month that the UN and African Union temporarily administer the country after fighting broke out once again.South Sudan's pushback comes as UN officials say the government has begun a crackdown that includes seizing dozens of passports of UN workers and imposing restrictions on travel and delivery of food aid.Deadly fighting in the capital, Juba, last month raised fears of a renewed civil war after an August 2015 peace deal and worsened a humanitarian crisis.Rebel leader and former first vice president Riek Machar fled during the fighting and says he will return only when regional peacekeepers secure the capital. An East African political body, IGAD, last week said South Sudan had agreed to a regional force, but Makuei on Wednesday disagreed and said the government had not been consulted.Under the US proposal, the regional force would report to the UN force that numbers more than 12 000 peacekeepers but has been criticised for not acting to protect civilians. The regional force would protect the airport and promote "safe and free movement" in and out of the capital.Seized UN passports"If South Sudan is turned into a UN protectorate, then this is not the end of the game but the beginning," the government spokesman, Makuei, said. "It will begin with South Sudan, but it will end up with all of us being turned into new colonies."The US ambassador to South Sudan, Molly Phee, said the US proposal is "entirely in line with what IGAD requested, and South Sudan is a member of IGAD."South Sudan's government has seized at least 86 passports of UN workers and imposed other restrictions that are a "clear violation" of the UN's operating agreement with the government, a spokesperson for the UN mission, Yasmina Bouziane said.South Sudan's foreign minister, Deng Alor, has called it a temporary safety measure.The UN also said the government has forbid it from travelling south of the capital.In addition, its World Food Programme had its flight clearances revoked for all food drops coming from neighbouring Ethiopia, said a spokesperson, George Fominyen. The UN has warned that millions in the country will face food shortage in the coming months.