Britain and France have said they are not ruling out any options in Syria, including a military-enforced no-fly zone to protect thousands of civilians fleeing the escalating civil war.
William Hague, Britain's foreign secretary, told a joint news conference in New York with Laurent Fabius, the French foreign minister, that a Turkish proposal for a safe zone would require military intervention.
"We're ruling nothing out and we have contingency planning for a wide range of scenarios," he said, speaking on Thursday ahead of a UN Security Council now under way to discuss how to ease Syria's humanitarian crisis.
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Asked whether the options included a NATO-enforced no-fly zone without security council authorisation, he repeated: "We are not ruling out any options.
"We also have to be clear that anything like a safe zone requires military intervention and that of course is something that has to be weighed very carefully."
Fabius said Paris and London were in "complete unity ... on this point".
Following the news conference, the UN said any proposals to set up to secure safe zones raised "serious questions" and
would need to be studied carefully.
"Such proposals raise serious questions and require careful and critical consideration," said UN Deputy Secretary-General Jan Eliasson.
France, which is president of the Security Council for August, convened Thursday's meeting, which is also being attended by ministers from Syria's neighbours Turkey, Lebanon and Jordan.
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"If Assad falls quickly, then the reconstruction can take place, but if sadly the conflict continues then we have to examine various solutions," said Fabius. "We have to be realistic."
The two countries announced an increase in their humanitarian aid - $4.74m from London and $6.25m from Paris - and called on other states to boost their commitments.
Fabius said Paris was channeling some of its aid to areas of Syria no longer under government control so that local communities can self-govern, encouraging people not to flee to neighbouring countries.
More than 200,000 Syrians, and as many as 300,000 according to some aid groups, have poured out of Syria since the uprising against Assad's rule began last year, while up to 3 million are displaced.
Turkey, which has seen the highest refugee influx, wants a quick solution to the problem.
Ahmet Davutoglu, the Turkish foreign minister, urged the Security Council to create refugee camps in safe zones in Syria, warning that it could not cope with the flow of people fleeing the conflict.
He said that more than 80,000 Syrians are now in camps in Turkey and his country faced serious difficulty coping with the 4,000 crossing the border each day and 10,000 waiting at the frontier.
"The UN should initiate the establishment of IDP camps within Syria without delay. Needless to say these camps should have full protection," he said.
He spoke after Britain, France and UN officials highlighted the diplomatic and legal obstacles that prevented the immediate creation of safe zones.
Their foreign ministers said the divided Security Council would not agree a resolution allowing the necessary military protection for safe zones.
Davutoglu slammed the Security Council for its failure to agree on international action on the 17-month-old conflict.
The Turkish minister also highlighted the absence of many ministers from the meeting which was called by France to discuss the humanitarian crisis in Syria.
'Assad a criminal'
While Thursday's meeting will focus on the humanitarian crisis, Fabius and Hague urged members of Assad's government and military to defect and renewed their call for the Syrian president to be held accountable before the International Criminal Court.
"Assad is a criminal and a criminal must be judged and punished," Fabius said.
However, the 15-member council is deadlocked over taking strong action after Russia and China blocked three resolutions that criticised Assad and threatened sanctions.
Less than half the council members are sending their foreign ministers, and of the permanent members - the US, China, Russia, Britain and France - only the latter two are attending.
Algerian diplomat Lakhdar Brahimi, who will replace Kofi Annan as the UN-Arab League Syria mediator on Saturday, will also attend but will not brief members.
Annan blamed the Security Council impasse for hampering his six-month-old bid to broker peace and leading to his decision to step down.