Britain, France, Spain and the United States had sought the measures to shore up efforts by UN envoy Bernardino Leon to clinch a political deal before the start of Ramadan on June 17.
The four countries asked that a global travel ban and an assets freeze be imposed on Abdul Rahman Swehli, a politician from Misrata, and Othman Maliqta, commander of the Zintani Qaqa brigade.
A new crucial round of talks is opening in Rabat on Monday.
Russia and China spoke out and said they would not back the proposed sanctions, diplomats said.
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The two Libyans are not considered high-profile, but the decision to target them first was aimed at sending a message to more senior-ranking officials that they must get onboard.
Libyan factions agreed during Geneva talks in January to set up a national unity government to restore stability that has been shattered since the 2011 fall of Moamer Kadhafi.
But months of UN-brokered negotiations have yet to produce a political accord.