Following reports of a large-scale chemical weapons strike, pressure is mounting on the United States and its Western allies to act to halt the violence in Syria.

But, with politicians and public alike loathe to get sucked into another drawn out land war in the Middle East, what military options remain on the table?

BUFFER ZONES

Some experts suggest buffer zones could be created along Syria's Turkish and perhaps Jordanian borders to serve as a safe area for refugees and as a rebel rear base.

This would limit the area that international troops would need to control but, as America's top general Martin Dempsey warned in a letter to US lawmakers, it would be no easy task.

"Lethal force would be required to defend the zones against air, missile, and ground attacks," the Chairman of the Joint Chiefs of Staff said.

"This would necessitate the establishment of a limited no-fly zone, with its associated resource requirements. Thousands of US ground forces would be needed, even if positioned outside Syria, to support those physically defending the zones."

Meanwhile, security expert Anthony Cordesman of the Center for Strategic and International Studies warned that the tactic might not be enough to ensure President Bashar al-Assad's defeat.

"It may well mean rebel defeat or giving the Assad regime control over so much of Syria that little is left to the rebels except for the equivalent of armed refugee camps at Syria's margins or borders," he said.

"They might also end up shielding Syrian refugees near the border area without offering any real hope for the future."

NO FLY ZONE

Influential voices, including some US lawmakers, have called for allied air power to enforce a no-fly zone over Syria in order to prevent Assad's regime from bombing rebel forces and civilian populations.

Senator John McCain, a senior figure on foreign policy and former naval aviator, has argued that this could be accomplished relatively easily, but other experts have warned that it would not be without risk.

According to a US Air Force study: "Syria's air-defense network at the start of the civil war ranked among the most capable and dense in the world, perhaps second only to North Korea's and Russia's.

"The overlapping coverage of missiles and radars consisted of approximately 650 static air-defense sites, the most worrisome of which housed the SA-5 Gammon, having a range of 165 nautical miles and an altitude capability of 100,000 feet."

The US Joint Chiefs of Staff warn that the campaign to establish a no-fly zone would not simply involve patrolling fighter jets, but also bombing runs against air defense sites and Syrian airbases, backed by refueling tankers and electronic countermeasures.

"We would require hundreds of ground and sea-based aircraft," Dempsey said.

Just closing down Assad's main air bases would require a salvo of at least 72 cruise missiles on the first night alone, according to a report by the Institute for the Study of War.

Cordesman said such a campaign would require access to airfields in the region, and support from US allies such as Britain, France and Arab Gulf monarchies.

And even with such a coalition, it might not have the desired effect, according to the retired US commander for the Middle East, General James Mattis.

"Can we do it? Absolutely," he said. "And the killing will go on on the ground because they are not using aircraft to do most of the killing."

DESTROY OR SECURE SYRIA'S CHEMICAL WEAPONS

Although most of the estimated 100,000 dead in the civil war have been killed with conventional arms, the international community has been outraged by Assad's alleged use of chemical weapons.

Western experts believe the regime holds hundreds of tonnes of sarin, VX and mustard gas, and some have called on American forces to lead an operation to destroy or secure the stockpile and prevent it being used on civilians or falling into terrorist hands.

Again, Dempsey was unenthusiastic.

"At a minimum, this option would call for a no-fly zone as well as air and missile strikes involving hundreds of aircraft, ships, submarines, and other enablers," he said.

"Thousands of special operations forces and other ground forces would be needed to assault and secure critical sites," he warned, adding that even this would offer no guarantees.

"The impact would be the control of some, but not all chemical weapons," he said, adding: "Our inability to fully control Syria's storage and delivery systems could allow extremists to gain better access."