The names of Machar, President Salva Kiir's army chief Paul Malong and his information minister Michael Makuei were on a sanctions blacklist circulated to the Security Council, diplomats said.
Under the proposed measure, Machar, who is receiving medical treatment in South Africa, and the two other officials would face a global travel ban and an assets freeze.
The sanctions list was presented after the United States late Thursday put forward a draft resolution on imposing an arms embargo on South Sudan, where a civil war has raged since December 2013.
Deputy Ambassador Petr Iliichev declined to specify today whether Russia would veto the draft resolution, but added that Moscow feels "very strongly" that the measure was ill-advised.
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"It's going to affect very negatively the small progress that we have achieved, especially on trying to re-start the inclusive process if we are going to sideline the leaders," Iliichev told reporters.
Russia last year blocked a previous bid by the United States to blacklist Malong along with top rebel commander, Major General Johnson Olony. Angola, China and Venezuela also opposed the move.
US Ambassador Samantha Power said yesterday that months of talks with South Sudan's leaders had failed to persuade them to opt for peace as she made the case for sanctions.
"There is no good reason why we would not deprive those who have shown a willingness to commit mass atrocities of the means of doing it more efficiently," she said.
The world's youngest nation, South Sudan descended into war in December 2013, leaving tens of thousands dead and more than 2.5 million people displaced.
A peace deal between Kiir and Machar in August last year had raised hopes of peace, until clashes erupted in Juba four months ago.
Malong was singled out in a report by a UN panel of experts as having ordered large-scale attacks during the violence that engulfed the capital in July, forcing Machar to flee the country.
Machar later called on his followers to take up arms against Kiir's government after a replacement was named in his post as vice-president and he was sidelined from the power-sharing deal intended to end the war.