Russia has threatened to veto a UN Security Council resolution on Syria if it is put to a vote on Friday, diplomats say.
Russian officials say they need more time to consult Moscow, BBC UN correspondent Barbara Plett reports.
Meanwhile, a new draft resolution is to be sent back to governments for further deliberation.
The wording has already been watered down in an apparent attempt to overcome Russian objections.
The new text drops explicit calls for President Bashar al-Assad to yield powers to a deputy to oversee a political transition.
His stepping down is a key part of an Arab League plan to end the Syria crisis, but Moscow insists such a move would amount to regime change imposed from outside.
Diplomats said Russian Ambassador Vitaly Churkin made the latest announcement at a closed-door meeting of the council.
Earlier on Thursday, some envoys said the council was close to agreement on the draft.
Despite the latest Russian objections, our correspondent says a vote-ready version of the revised text will be prepared and sent to capitals later on Thursday.
It is hoped that high level decisions can then be taken that would allow a vote as soon as possible, she adds.
Colombian UN ambassador Nestor Osorio told journalists that talks would continue on Friday.
The UN Security Council began negotiations on Wednesday after a high-level meeting urged the council to back the Arab League plan.
Russia, a key ally of Syria, has continued to supply weapons to the Assad government despite the uprising.
Human rights groups and activists say more than 7,000 people have been killed by Syrian security forces since the uprising began in March.
The UN stopped estimating the death toll in Syria after it passed 5,400 in January, saying it was too difficult to confirm.
The government says at least 2,000 members of the security forces have been killed combating "armed gangs and terrorists".