Russia has proposed that the United Nations Security Council blacklist Syrian rebel groups Jaish al-Islam and Ahrar al-Sham for links to Islamic State and al-Qaeda militants.
If none of the council's 15-member Islamic State and al-Qaeda sanctions committee blocks or puts a hold on the listing by 19:00 GMT on May 11 then the groups will be added to the U.N. sanctions list, said diplomats.
"The reason for such a move was the information that these groups, which are waging a war in Syria, are closely connected to terrorist organizations, first of all with ISIS [Islamic State] and al-Qaeda," Russian U.N. Ambassador Vitaly Churkin said in a statement on Tuesday.
A senior Security Council diplomat, speaking on condition of anonymity, described the Russian move as "unhelpful".
"It's a way of trying to divide the opposition," the diplomat said on Wednesday.
Jaish al-Islam (Army of Islam) is a major armed rebel group in Syria and part of the High Negotiation Committee, which was set up in Riyadh last December to negotiate on behalf of opposition groups at U.N.-brokered peace talks with the government.
The High Negotiation Committee is backed by Western nations and key Arab states.
Ahrar al-Sham withdrew from the Riyadh meeting, saying "revolutionary groups" were sidelined. But the group did attend the latest round of peace talks in Geneva.
Russia's Foreign Ministry has long said that Jaish al-Islam and Ahrar al-Sham should not be involved in Syria peace talks.
Ahrar al-Sham is an ultra-orthodox Salafist group and has fought as part of a military alliance including the al-Qaeda-linked Nusra Front, which was not part of a cessation of hostilities agreement brokered in February.
Ahrar al-Sham, whose late leader fought alongside Osama bin Laden, last year denied sharing al-Qaeda's ideology or having organizational ties to the group.