Despite the escalating violence in Syria, Western powers refuse to unleash a Libya-type operation against a regime backed by a much bigger army and powerful allies.

In Washington, Paris, London and NATO headquarters, leaders have sharpened their tone against President Bashar al-Assad but they have refrained from beating the war drums, favouring instead the path of sanctions and diplomacy.

"Let me stress, we have no intention whatsoever to intervene in Syria," NATO Secretary General Anders Fogh Rasmussen repeated last week.

US Secretary of State Hillary Clinton has called for Assad to step down but she also ruled out a military intervention after Russia and China vetoed a UN resolution aimed at ending the violence last week.

Two hawkish senators, John McCain and Joe Lieberman, said President Barack Obama's administration should step up its involvement in the crisis by providing weapons to the rebel Syrian Free Army.

But the White House rejected the idea and said it was exploring the possibility of providing humanitarian aid to Syrians.

"Choosing the path of arms deliveries in an official and aggressive way would be planting the seeds of a civil war, and this is exactly what we want to avoid at all cost," said a senior European diplomat.

NATO member Turkey, an ally-turned-critic of Assad, has also denied making any contingency plans for any military intervention in the neighbouring nation.

Russia and China, critics of the scope of the Libyan operation, likely blocked the UN Security Council resolution on Syria for fear it would encourage another intervention, diplomats said.

"One of the real political challenges we run into at the moment, and which is at the root of the veto, is that any reminder of the Libyan precedent raises reservations," the European diplomat said.

This time, however, Europeans and Americans have repeatedly ruled out military action in Syria, stressing that the situation was different.

Turkey and other nations are hoping to form a "friends of Syria" group to intensify diplomatic efforts instead.

In Libya, the Arab League backed a no-fly zone, the UN endorsed the intervention and there was an urgent need for action, officials say.

An air bombing campaign also presents many risks in a country much more densely populated than Libya, defended by an army of nearly 300,000 active troops and armed with thousands of tanks and missiles.

"Syria is not Libya," said Aram Nerguizian, an analyst at the Center for Strategic & International Studies in Washington, in a report assessing the risks of a military intervention in Syria.

"While the latter may be geographically much larger, it is a mostly empty country with a small population and very limited military capacity," he said.

Noting that it took seven months of air bombings for Moamer Kadhafi to lose power in Libya, a senior European military official commented: "How long would it take for Assad, who is much stronger?"

With allies like Iran and Lebanese militant group Hezbollah behind Syria, a Western intervention could also spark a regional conflict that could suck in Israel.

"Iran has a crucial stake in Assad's continuing rule as this affords them and their proxies (i.e., Hezbollah) room for operation, which may be denied them in any future Syrian configuration," said David Roberts, expert at the Royal United Services Institute in London.

US Senator John Kerry, a Democrat who chairs the Senate foreign relations committee, has also highlighted the differences between Libya and Syria.

"This is a very different playing field, very different set of players, very different set of possible prospects," Kerry said.

"I think we have to approach it differently. I think we have to condemn what is happening -- and we have. I think we have to work very diligently with China and Russia to see if we can move them, change their positions."