Diplomats say Russia and China blocked US attempts to get the UN Security Council to condemn violence and Sudanese government bombing in the South Kordofan region of Sudan.
South Kordofan lies across the border from newly independent South Sudan, and the clashes between government troops from Sudan's Arab north and forces aligned with the south have added to the strained relations between the two countries.
The US circulated a statement which would have condemned the violence and called for an end to the aerial bombings, but the diplomats, speaking on condition of anonymity because discussions were private, said the US withdrew it on Friday.
US Mission spokesman Mark Kornblau said the situation demanded a "strong response ... not a watered-down statement."