To that end, Iran -- Syria's other major regional ally -- has been using Iraqi airspace to fly small arms, infantry weaponry and personnel into Syria, according to U.S. officials.

Iraq says it is conducting random searches on Syrian-bound Iranian planes that use its airports, but as Binnie points out, "the Iraqis don't have any way to force Iranian planes to land."

In addition to the tanks and troops fanned out across the country, Syria has also deployed communications interception systems to try to track rebels. CNN reporters who have been on the ground there say for the most part using a cell phone is out of the question, as Syrian forces can easily triangulate the user's location.

What weapons do Syrian rebels have? Where are they coming from?

The rebels are severely outmatched, and most of their weaponry was either taken from Syrian military arsenals or obtained from local black markets.

In addition to individual infantry weapons like AK-47s, tripod-mounted PK machine guns, and RPG-7 grenade launchers -- the ubiquitous shoulder-mounted weapons seen on nightly news reports from conflicts around the world -- Binnie says rebels have also been seen with Strela-2 shoulder-launched surface-to-air missiles.

Rebels have also commandeered Syrian tanks, but since they don't generally have the capacity to maintain and refuel them, according to Binnie, they've instead been stripping off the heavier guns and mounting them on civilian vehicles.

Binnie says the increasing number of improvised weapons and explosives being used in Syria shows the rebels, without a foreign power to supply them, may be struggling to maintain adequate levels of ammunition.

"Some of the weapons in Syria look very similar to what we've seen in the last year in Iraq, where they've finally ran out of all the ordinates that were lying around from the 2003 U.S. invasion and have had to improvise," he told CNN.

What other groups are operating in Syria?

Charles Lister, an analyst at IHS Jane's Terrorism and Insurgency Centre, says militant groups "of all persuasions" are now operating in Syria -- and that some groups increasingly appear to be carrying out joint co-ordinated attacks.

While Lister says the majority of the militias now in Syria are not Islamic extremists, analysts believe a hard-line jihadist group known as Jabhat al Nusra, which has claimed responsibility for a string of recent suicide attacks across Syria, has close links with al Qaeda's affiliate in Iraq.

"Their focus now is on recruiting suicide bombers. They want to copycat the Zarqawi model," Noman Benotman, a former Libyan Jihadist now with the Quilliam Foundation in London, told CNN.

The "Zarqawi model" refers to the devastating campaign launched by al Qaeda in Mesopotamia leader Abu Musab al Zarqawi after U.S. troops occupied Iraq.

At the same time, Kurdish militias now control at least 10 towns and cities in north-eastern Syria, some near the Turkish border -- an issue Lister says is "of significant concern to Ankara", which has been battling Kurdish separatists along its borders for decades.

Is Assad losing control of the Syrian army?

Not yet, it seems. Despite some high profile defections, Binnie says "we haven't seen significant Syrian army units going across to join the rebellion."

Binnie says: "The speculation is that the Syrian army is focused on keeping itself together more than deploying some of the potentially less reliable units into rebel held areas."

Like most of the nation itself, the majority of Syria's conscripted army is Sunni Muslim. Rather than send some less-than-keen battalions in to kill their own countrymen, Binnie says the approach may be to have the army bombard cities from afar, before sending in loyal militias to do the up-close fighting street to street.

"You give the militias the weapons and the mobility, and you just make sure that the army units which are largely Sunni conscripts just stay together," he told CNN.