A confidential letter from U.N. Secretary-General Ban Ki-moon says South Sudan's government is undermining a U.N. Security Council decision to send an additional 4,000 peacekeepers into this troubled nation, risking an arms embargo as fighting continues.

The letter dated Monday also lists a number of government obstructions to the U.N. peacekeeping mission in this East African country where civil war erupted in late 2013, including the demand that some U.N. aid convoys include South Sudanese soldiers.

Under a resolution the council approved in August, such a letter from Ban is meant to trigger a council meeting within five days to consider imposing an arms embargo and other options.

After a similar letter last month, the council discussed South Sudan behind closed doors but reportedly remain divided on an arms embargo.