Western powers have few military options in Syria but limited long-range missile strikes may be the best way of making President Bashar al-Assad pay for his alleged use of chemical weapons.

The United States, Britain and France have all warned that they cannot let pass unchallenged a poison gas attack last week in a Damascus suburb which has left hundreds of people dead, according to medical and opposition sources.

However, military action is full of risk and the emphasis has to be on avoiding escalation in a conflict which the United Nations says has already cost more than 100,000 lives and which has stoked sectarian tensions across the region, analysts said.

"We are in a situation where... no Western power wants to intervene," said Vivien Pertusot, head of the French Institute of International Relations in Brussels.

"But the use of chemical weapons, if confirmed, forces them to intervene, to do something," he said as UN experts headed for the site in Damascus on Monday.

Top US military officials, however, have warned repeatedly of the danger of getting sucked into the conflict in Syria, arguing that no-fly-zones and attacks on Assad's chemical weapons sites would need a long-term and costly commitment.

Short of that sort of involvement, it is most likely the United States and its close allies will rely on long-range cruise missiles to target Assad's military assets, analysts said.

"The objective is punitive, not regime change, not to tip the balance in favour of the rebels," said Jonathan Paris, a senior fellow at the Atlantic Council think-tank.

The West must act but must also make clear this is a limited intervention, designed to punish Assad and protect civilians, Paris said.

This will say that "never again that you can get away with an unprovoked chemical attack on civilians," he said.

"If you don't do this, then it gives a licence to Assad and to any other dictator."

Markus Kaim at the Berlin-based SWP think-tank, said no-fly and buffer zones were "too risky" while the object had to be to convey a political message to Assad.

The "only realistic option is attacks from the sea against amunition depots and the command structure of the Syrian army," Kaim said.

Long-range missiles fired from US planes could also be used, to minimise the risk to US or allied pilots from Syria's formidable air defences, analysts said.

That has to be a key consideration -- heavy loss of aircraft and crew could quickly test the resolve of a West grown tired of the "global war on terror" and the bloody, inconclusive wars in in Iraq and Afghanistan.

Assad made no empty threat Monday when he said Washington faced failure if it attacked Syria, as it had in Vietnam.

"Yes, it is true, the great powers can wage wars but can they win them?" he asked defiantly.

Russia's blunt warning of the "extremely dangerous consequences" of Western military action highlighted the political and strategic stakes involved.

Both Paris and Pertusot said the West would not need to seek UN approval -- it might provide welcome political cover but Russia would almost certainly veto it -- given the precedent of the US-led bombing campaign in Kosovo in 1999 to force the withdrawal of Serb forces.

Significantly, US officials were looking at the Kosovo operation as a possible blueprint for strikes on Syria without a UN mandate.

The administration would also need to weigh up the possible effects that a bombing campaign on Syria would have on immediate neighbours countries such as Lebanon, Jordan, Turkey as well as Egypt.