UN Secretary General Ban Ki-moon says weapons inspectors investigating an apparent Syrian chemical attack will leave on Saturday and will then submit their report to him.
Hundreds are reported to have died in the attack near Damascus last week.
US President Barack Obama said he had not yet decided on a plan of action.
Other nations are also considering the next move. The UK wants a UN Security Council resolution to take “all necessary measures” to help civilians.
The UK prime minister’s office said in a statement that the UK could still take “exceptional measures including targeted military intervention” even if the Security Council could not agree.
Syrian President Bashar al-Assad said his country would defend itself against any aggression.
The British parliament is voting later on Thursday on whether to back the principle of military intervention, but the leader of the opposition Labour Party has said MPs should not have to decide on what he called an “artificial timetable”.
The Speaker of the Syrian parliament has written to his counterpart in London inviting a British parliamentary delegation to visit Damascus as soon as possible.
French President Francois Hollande has also yet to decide about a military intervention. But on Thursday, after meeting Ahmed Jarba, the head of the opposition Syrian National Coalition, Mr Hollande said a political solution would only be possible if “the international community can put a temporary stop to this escalation in violence”.
Meanwhile, a Chinese state-run newspaper has warned there are no excuses for air strikes on Syria.
The China Daily editorial accuses Western powers of acting as judge, jury and executioner before the UN has completed its investigation.
Russia, President Assad’s main international ally, also says it opposes any foreign military intervention in Syria.
The use of force without a sanction of the UN Security Council would be a “crude violation” of international law and “lead to the long-term destabilisation of the situation in the country and the region”, Mr Lavrov said.
The US state department criticised “Russian intransigence” and said it could not allow diplomatic paralysis to serve as a shield for the Syrian leadership.
The UK, US and France are continuing their discussions following the meeting of the five permanent members
