France has said the ceasefire in Syria is not working and has suggested military action may be the next step needed to stop the violence there.
French foreign minister Alain Juppe said the UN-backed peace plan has been "strongly compromised" and that monitors should be on the ground in a fortnight, not three months.
Without quick progress, Mr Juppe said the international community would have "to move on to another step which we have already started raising with our partners, under Chapter Seven of the United Nations charter".
A Chapter Seven resolution, which can be imposed by the UN Security Council if member states think peace is threatened by an act of aggression, authorises foreign powers to take measures, including military options.
After a meeting with Syrian opposition members the French politician said: "The Damascus regime does not respect the commitments it made. Repression is continuing. Monitors cannot work on the ground. This cannot last indefinitely."
President Bashar al Assad's crackdown on anti-government protests has continued for over a year and thousands of people have been killed.
Mr Juppe added that if the UN mission "is not working, we cannot continue to accept the defiance of the regime" and the international community will have "to move on to other things to stop the tragedy".
UN-Arab envoy Kofi Annan had urged a rapid deployment of the full, 300-strong observer team agreed by the Security Council .
Mr Juppe said he hoped Russia, which has blocked some previous efforts against the Syrian regime in the Security Council, would draw the right conclusions from Syria's efforts to block the monitors' deployment.
It comes as at least 27 civilians were killed across the country, taking the total who have died since a ceasefire went into effect on April 12 to around 300.
Twelve of them died in government shelling of the Mashaa al-Tayaran district in the central city of Hama, the London-based Observatory said.
Elsewhere, two civilians were killed by sniper fire in Douma, a northeastern suburb of the capital Damascus.
It was unclear whether UN monitors, who visited Douma on Wednesday, were present before or afterwards.
Three soldiers died in clashes with armed rebel groups in the southern province of Daraa, cradle of the uprising against President Bashar al-Assad.
Mr Annan has branded the bloodshed "unacceptable".
"We need eyes and ears on the ground, able to move freely and quickly, and to engage all parties - something which must be guaranteed by the Syrian authorities," he said.
But UN peacekeeping chief Herve Ladsous said it would take at least one month to get the first 100 into Syria.
He told the Security Council Damascus was refusing to accept monitors from the Western and Arab coalition of countries in the so-called Friends of Syria group that has backed the Syrian opposition.
Addressing the UN Security Council via teleconference, Mr Annan said he was "concerned" about the violence surging after observers visit individual cities.
The former UN chief said President Assad has still not fulfilled a promise to end violence and said the situation was "bleak" and "unacceptable".