"In response to this continued violence and brutality against civilians and humanitarian workers, the United States is enacting restrictions on arms transfers with South Sudan," State Department spokeswoman Heather Nauert said in a statement.
The United States is South Sudan's biggest aid provider, and was a major backer of its 2011 independence from Sudan.
But patience from South Sudan's foreign allies has worn out after countless failed efforts to bring peace to a country, now in its fifth year of a war where targeted ethnic killings, gang rapes and other atrocities have occurred.
Roughly four million South Sudanese have been forced to flee their homes.
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"The government and armed opposition have continued offensive military actions, and the government obstructs the UN peacekeeping mission from fulfilling its mandate," Nauert said, adding that at least 95 aid workers have been killed since the conflict began in December 2013.
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 seeking a UN Security Council arms embargo on all weapons flowing into South Sudan.
"The message must be clear -- the United States, the region, and the international community will not stand idly by as innocent South Sudanese civilians are murdered," Nauert said.
In November, Washington had threatened to take unspecified measures against President Salva Kiir's government.
The threat followed a visit by UN Ambassador Nikki Haley to the capital Juba in October, where she held talks with Kiir and became the highest level US administration official to visit South Sudan.
The US imposed sanctions on three senior officials in September last year.
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