France and Britain have warned Syrian President Bashar al-Assad that military action to secure safe zones for civilians inside the country was being considered.
The warning came despite the paralysis of the UN Security Council over how to end the 17-month conflict.
While the Security Council impasse between western nations and Russia and China means a resolution to approve such a move appears impossible, countries could act outside the authority of the world body and intervene, as happened in Kosovo in 1999.
"We're ruling nothing out and we have contingency planning for a wide range of scenarios," British Foreign Secretary William Hague told a news conference at the United Nations.
He was speaking ahead of a meeting of Security Council foreign ministers to discuss how to ease Syria's humanitarian crisis.
"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," Mr Hague said.
Turkish Foreign Minister Ahmet Davutoglu, who is attending the meeting, urged the United Nations yesterday to protect displaced Syrians inside their country, but Assad dismissed talk of a buffer zone.
Creating a buffer zone for displaced Syrians would be difficult because a UN Security Council resolution would be needed to set up a no-fly zone to protect the area, and Russia and China would not approve such a move, diplomats said.
It is not the first time Russia has posed difficulties for the United States and its allies on the Security Council.
In the 1990s, Moscow strongly supported Serbia in the Balkan Wars and acted as Belgrade's protector on the council.
After an ineffectual UN presence failed to stop genocide in the 1992-1995 Bosnian War, the United States and its European allies infuriated Russia by bypassing the deadlocked Security Council and turning to NATO to halt the Serbian onslaught in Kosovo with a bombing campaign against Serbia in 1999.
As Syria spirals deeper into a civil war, the 15-member council is paralyzed as Russia and China have blocked three Western-backed resolutions that criticized Syrian President Bashar al-Assad and threatened sanctions.
France, which is council president for August, had hoped the body could unite to deal with a shortfall in humanitarian aid and convened today’s meeting, which will also be attended by ministers from Syria's neighbours Turkey, Lebanon, Jordan and Iraq.
"If Assad falls quickly, then the reconstruction can take place, but if sadly the conflict continues then we have to examine various solutions. We have to be realistic," French Foreign Minister Laurent Fabius told the joint news conference with Mr Hague.
But the absence of the US, Russian and Chinese foreign ministers at the meeting highlights the Security Council's failure to end Syria's conflict, which the UN says has killed nearly 20,000 people.
Less than half the council members have sent ministers, and of the permanent members - the United States, China, Russia, Britain and France - only Mr Fabius and Mr Hague are attending.
The two countries announced an increase in their humanitarian aid - £3m from Britain and €5m from France - and called on other states to boost their commitments.
Diplomats said the meeting would not produce any further action on Syria from the Security Council.
"We wanted a resolution on humanitarian issues, but we faced a double refusal," said a French diplomat, who did not want to be identified.
"The United States and Britain believe we have reached the end of what can be achieved at the Security Council, and Moscow and Beijing said that such a resolution would have been biased."
Fabius said Paris was channelling 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 Syria to neighbouring countries.
Syrian civilians killed while queuing to buy food
Earlier, Human Rights Watch accused Syria’s military of targeting civilians with attacks on bakeries in Aleppo.
Syrian jets and artillery have struck at least ten bakeries in Syria’s largest city in the past three weeks, killing dozens of people as they waited in line to buy bread, HRW said.
The US-based group said the attacks were either aimed at or were done without care to avoid the hundreds of civilians forced to queue outside a dwindling number of bakeries in Aleppo, a front line in the civil war.
"The attacks are at least recklessly indiscriminate and the pattern and number of attacks suggest that government forces have been targeting civilians," HRW said.
"Both reckless indiscriminate attacks and deliberately targeting civilians are war crimes."
One attack on 16 August killed around 60 people and wounded more than 70, said HRW, which sent a researcher to the embattled city.
Food shortages in Aleppo, a focal point of the 17-month-old uprising against President Bashar al-Assad, have forced many bakeries to close, meaning huge queues for the food staple outside the remaining shops.
"Day after day, Aleppo residents line up to get bread for their families, and instead get shrapnel piercing their bodies from government bombs and shells," said Ole Solvang, the HRW researcher who visited Aleppo.
"Ten bakery attacks are not random - they show no care for civilians and strongly indicate an attempt to target them."
Thousands of rebels from Aleppo's countryside began moving into the city, Syria's economic hub, in July.
Many moved their fighters into schools and other buildings in residential neighbourhoods, leading to high civilian casualties as President Assad's forces pounded rebel-held areas with air strikes and artillery.
HRW said in five of the cases it investigated, there was no military target near the bakeries other than a few fighters maintaining order in the bread lines, meaning the areas were "clearly civilian objects".
"Every pilot who deliberately launches a rocket at a bread line of civilians, and every commander who gives such an order, should face justice for their crimes," Mr Solvang said.