By Peter Apps and Myra Macdonald
WASHINGTON/LONDON (Reuters) - - If the United States and allies genuinely want to change the course of the war in Syria, it may take considerably more than simply supplying the faltering opposition with weaponry.
Western officials say they still believe the ultimate endgame - and possibly the exit of Bashar al-Assad - will be through a negotiated settlement.
In the meantime, however, they say the war is increasingly tilting in what they see as the wrong direction, with Iranian-backed Hezbollah fighters entering the fray on Assad's side. With government forces advancing on Aleppo, time may also be running out.
That, some analysts and officials believe, may now push Washington and others towards much more direct military action, perhaps targeted airstrikes or a limited "no-fly zone" over rebel held areas near the Jordanian border.
What happens next may come down to a personal decision for President Barack Obama.
Russian opposition makes a Libya-style U.N. resolution authorizing force seem unlikely, but Britain, France and others are increasingly signaling they might be willing to act anyway given a strong U.S. lead, like in the Kosovo war in 1999.
The Obama administration has yet to fully explain what it meant when it announced on Thursday that Bashar al-Assad's limited use of chemical weapons had prompted it to decide to extend "military support" to the opposition.
In a conference call, Deputy National Security Adviser Ben Rhodes refused to specify what kind of support might be made available - although he said one key priority was to help ensure the rebels could act in a more cohesive, coordinated manner.
"We have not made any decision to pursue a military operations such as a no-fly zone," Rhodes said. "We still believe that the best thing we can do in terms of affecting the situation on the ground is to strengthen the opposition."
A no-fly zone, he said, would bring with it "considerable open-ended costs to the United States and the international community" and would be far more complex than in Libya.
He said the United States had contingency plans for much more direct military action - but specifically ruled out "boots on the ground" - a ground campaign involving U.S. troops.
The White House statement, some analysts say, may have been as much about signaling to Assad as anything else - warning him further chemical weapons usage would almost certainly spark much more serious action.
Western officials say that some of the equipment requested by the opposition - particularly portable missiles that can shoot down planes - remains off the table, given worries over growing Islamist influence and fears they might end up in the wrong hands. But it is not clear whether the weapons the West would be willing to send would be enough to tip the balance.
"Small arms and ammunition will certainly not change the military balance on the ground inside Syria," says Mona Yacoubian, a former US State Department official now Syria analyst at the Stimson Centre in Washington DC. "The provision of sophisticated, heavy weapons could begin to change (that), although that option is fraught with risks."
The use of targeted airstrikes, she said, would be a bolder and perhaps more effective strategy.
Syria's largely Russian-built air defenses, Western officials and defense analysts say, would be amongst the best Washington and its allies have faced since the 1999 Kosovo war - even without the more potent S-300 missile system Moscow has threatened to supply to make any foreign intervention harder.
Nevertheless, three Israeli bombing raids in recent months have shown that it can be penetrated.
U.S. B2 Stealth bombers and Tomahawk cruise missiles could penetrate Syrian airspace with relative impunity - but a more concerted no-fly zone would probably require an onslaught in its first few days to destroy radar and missiles.
Whether the United States and allies have the appetite for that kind of operation is far from clear.
The United States has considerable air power in the region, above all at its airbase in Incirlik, Turkey. There are two aircraft carrier groups in the Arabian Sea and one a short sail away in the Atlantic.
In the past weeks Washington also sent F-16 jets, Patriot missiles and more than 4,000 troops to Jordan for a military exercise, saying it might leave the assets in place.
"NO-FLY ZONE LITE"
Some current and serving officers and officials have long suggested such weaponry could be used to enforce a "no-fly zone-lite", firing anti-aircraft missiles into Syria from Turkish and Jordanian airspace to stop Assad's aircraft attacking rebel areas near the two respective borders.
Syria's radar coverage is widely seen as patchy, concentrated around the Russian naval base at Tartus and its border with Israel, making intervening in some areas easier than others.
A "safe zone" pushing some 20 km inside Syria might, some experts say, be relatively easy to defend - although it might require air strikes on Assad's ground forces to stop them moving forward in the zone.
Preventing Assad's forces from moving on Syria's second city of Aleppo, however, is seen much tougher. In Libya, a handful of French airstrikes proved enough to stop the forces of Muammar Gaddafi from taking Benghazi. Syria, however, is a much more geographically complex country with many more roads and routes.
With fighting increasingly close to the Turkish border, attempting to enforce a no-fly zone there might quickly degenerate into outright warfare with Syria. Even if it were successful in grounding Assad's planes and helicopters, Saddam Hussein demonstrated in 1991 that it was entirely possible to ruthlessly crush an uprising under U.S.-controlled skies.
Many of Assad's anti-aircraft and artillery units are also located close to civilian areas, raising the prospect of collateral damage. Some Western officials also worry Russian advisers and personnel might be caught in strikes, ratcheting up an already worsening proxy war with Moscow.
"It's pretty clear the U.S. has only been dragged to this stage kicking and screaming," said David Hartwell, a former British Ministry of Defense official and now Middle East analyst at IHS Jane's. "You could argue the fact that it's come to this is basically a sign that their previous policies - or lack of them - have simply failed outright."
(Writing by Peter Apps; Editing by Peter Graff)