The United States has banned the export of weapons to South Sudan and has urged other nations and the UN to adopt a global arms embargo.
As a result, the State Department will amend regulations and apply "a policy of denial, with limited exceptions, on the export of defense articles and defense services to South Sudan, including all parties involved in the conflict", she said.
"The United States is appalled by the continuing violence in South Sudan", spokeswoman Heather Nauert said in a statement.
The department said it hopes the restrictions will put pressure on South Sudan President Salva Kiir to end the country's civil war, which began in December 2013. Humanitarian partners in South Sudan need US$1.7 billion to provide life-saving humanitarian assistance to six million people across the country. Mr Machar is now in exile.
Thousands of men, women and children have been subjected to unimaginable acts of violence, including sexual assault, by government and opposition forces, some of which could amount to crimes against humanity or war crimes.
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The United States in the last days of the Obama administration tried and failed to have the U.N. Security Council impose an arms embargo on South Sudan, to the disappointment of arms researchers and rights groups who say the country is awash in weapons.
The move was backed by seven of the 15 Security Council members, including Britain and France, but received eight abstentions, including one from Russian Federation, whose top United Nations ambassador argued an embargo would not stabilize the country. Russian Federation has said it would only worsen the situation and China has said the U.N.'s most powerful body should send out more "positive and enthusiastic messages".
However, after five years of civil war, tens of thousands of deaths, and almost three million displaced refugees the U.S.is changing its stance.
The unilateral arms embargo is another signal by Washington that it is losing patience with South Sudan's leaders after ceasefires have been repeatedly violated. Both sides have been accused of restricting the delivery of aid to millions across the impoverished country, including an estimated 1.5 million people near starvation. This week the African Union joined the calls for further sanctions on those blocking the path to peace.