By Michelle Nichols

UNITED NATIONS Dec 22 (Reuters) - The U.N. Security Council is expected to vote on Friday on whether to impose an arms embargo on South Sudan, even though the U.S.-drafted measure is likely to fail despite warnings by U.N. officials of a possible genocide, diplomats said on Wednesday.

The resolution also proposes blacklisting South Sudan opposition figure Riek Machar, army chief Paul Malong and Information Minister Michael Makuei by subjecting them to an asset freeze and travel ban.

To be adopted, a resolution needs nine votes and no vetoes. Diplomats say that so far seven members were in favor, with the remaining eight planning to abstain. The United States has requested that a vote be held on Friday.

"Council members will need, each of us, to own our decisions. So the United States urges you to prepare to vote your conscience, and to vote to stand with the people of South Sudan," Samantha Power, the U.S. ambassador to the United Nations, told the council on Monday.

The United States has been unable to win over its ally Japan, which last month deployed troops to a U.N. peacekeeping mission in South Sudan. It is focusing lobbying efforts on Senegal and Angola, diplomats said.

"We urge all our council colleagues to vote in favor of that resolution on Friday. I don't know whether enough of them will do so," British U.N. Ambassador Matthew Rycroft told reporters on Wednesday.

Political rivalry between South Sudan President Salva Kiir, an ethnic Dinka, and Marchar, his former deputy, led in 2013 to civil war that often has followed ethnic lines.

The pair signed a peace deal last year, but fighting has continued. Machar, a Nuer, fled in July and is now in South Africa.

U.N. Secretary-General Ban Ki-moon on Monday told the Security Council that he feared genocide was about to start in South Sudan unless immediate action is taken, renewing his months-old plea for an arms embargo.

U.N. peacekeepers have been in South Sudan since the nation gained independence from Sudan in 2011, and there currently are some 13,700 U.N. troops and police in the country. (Reporting by Michelle Nichols Editing by W Simon)