U.S. Ambassador Nikki Haley says Washington is giving up on South Sudan President Salva Kiir after investing over $11 billion since the country's independence in 2011, calling him "an unfit partner" in the pursuit of peace and urging an arms embargo on the conflict-wracked nation.

She cited Kiir's failures to adhere to a Dec. 21 cease-fire and allow unhindered aid to millions in need as well as his promotion of three generals under U.N. sanctions for leading "the slaughter" of civilians.

In a hard-hitting speech to the Security Council, Haley said Wednesday that attempts to ease the suffering of South Sudan's people are failing "not despite the leadership of South Sudan, but because of it."

She accused South Sudan's leaders of "not just failing their people" but "betraying them."