The United Nations Security Council is due to vote on Friday on a US-drafted resolution that would impose an arms embargo on South Sudan, five years after civil war broke out in the country.
Diplomats made this announcement on Thursday.
A resolution needs nine votes in favor and no vetoes by Russia, China, Britain, France or the United States. Diplomats said they believed the United States had enough votes for the measure to pass and that it was unlikely to be vetoed.
Some council members say the timing is not right given attempts to revitalize the peace process and are likely to abstain, diplomats said.
A US bid to impose an arms embargo in December 2016 failed to get enough votes.
The US mission to the United Nations was not immediately available to comment.
South Sudan, which split off from its northern neighbor Sudan in 2011, has been gripped by a civil war since 2013 caused by political rivalry between President Salva Kiir and his former deputy Riek Machar.
Last Friday, the government and opposition signed an agreement on security arrangements which follow on from a ceasefire deal last month.
However, on Thursday South Sudan's Parliament voted to extend Kiir's mandate until 2021 in a move likely to undermine the peace talks as opposition groups have said the change would be illegal.
At the end of May, the Security Council renewed its targeted sanctions regime on South Sudan until July 15 and said it would consider an arms embargo and blacklisting six senior South Sudan officials if UN chief Antonio Guterres reported by June 30 that there was still conflict or a lack of a viable political agreement.
That resolution was adopted with nine votes in favor and six abstentions.
"There have been credible reports of fighting," Guterres told the Security Council in a June 29 letter. He also said UN peacekeepers had documented gross violations of international human rights and humanitarian law.
"While the outcome of regional and international efforts to deliver a political settlement of the conflict is yet unclear, I must reiterate that any revitalised agreement must be inclusive, fair and sustainable," Guterres wrote.
The UN human rights office said on Tuesday that at least 232 civilians were killed and 120 women and girls raped in "scorched earth" attacks by SouthSudan government troops and aligned forces in opposition-held villages earlier this year.