* Bombs give rebels more bang for buck
* Tactical switch in response to army strength
* Price of guns in region has soared
BEIRUT Syrian rebels fighting to topple President Bashar al-Assad say they are shifting tactics towards homemade bombs, hoping to even the odds between their outgunned forces and his powerful army.
A series of deadly blasts in the past week suggests they are getting better at it.
Suicide bombs, booby-trapped cars and roadside explosions, including blasts in Idlib on Monday and the capital Damascus last week, have rocked the Arab state. The attacks threaten to sour the UN-brokered two-week truce and have killed many from Assad's security agencies.
"We are starting to get smarter about tactics and use bombs because people are just too poor and we don't have enough rifles," a rebel fighter from the north of Idlib province said last week as he took a break across the border in Turkey.
"It is just no match for the army," said the man, who spoke on condition of anonymity, "So we are trying to focus on the ways we can fight."
Details of what disparate groups are doing inside Syria are sketchy because the government bars most independent media.
The bombings have produced an array of theories, including that some may be self-inflicted wounds by security agents out to discredit the rebels, or that they show the rise of al Qaeda-linked Syrian Islamists, of whose expertise there is no doubt after their years of activity across the border in Iraq.
However, mindful of Assad's portrayal of those who have opposed him over the past 14 months as "terrorists", and keen to maintain Western and Arab support, several rebel fighters who spoke to Reuters said that, unlike al Qaeda, their bombs were aimed at military, and never civilian, targets.
"We are not targeting civilians. We are strictly going against regime targets," said Haitham Qdemati, spokesman for a rebel group called the Syrian Liberation Army. "We're not killers. We're defending ourselves."
BOMBS VS BULLETS
The Free Syrian Army (FSA), which lays claim to overall command of rebel forces but lacks the means to control them, says it has nothing to do with the bombings and is sticking to the U.N.-brokered ceasefire. In 18 turbulent days, the truce has been jeopardized by army shelling and insurgent attacks.
But some fighters have rejected the truce. They say it cannot prevent a slide into civil war against a ruling elite that has no intention of bargaining away a dominance the rebels challenged first with street protests and now with armed rebellion.
Although an obscure Islamist group claimed recent suicide bombings in Damascus, many rebel fighters say their switch in tactics from guns to bombs is down to economics, not ideology.
Firefights and skirmishes are expensive for the ragtag rebel force, many of them young men from impoverished rural areas who have scrounged cash and weapons from sympathizers abroad.
Rebels say the price of rifles and ammunition smuggled from neighboring Lebanon and Iraq has skyrocketed. A Russian-made AK-47 can go for $2,000 with bullets at more than $4 each - several times the normal price in open markets. In the United States, the same gun costs under $400 and bullets about 30 cents.
"Buying chemicals in grocery stores or even smuggling in equipment is cheaper than getting weapons and we can do more with it once we improve our skills," said another rebel from northern Idlib province, who called himself Mustafa.
"We have a lot of guys who devote their time to this."
Some of these bombing skills may have been brought back from fighters who joined the Sunni insurgency in neighboring Iraq against the U.S. occupation forces. The presence of hardliners from a Syrian Sunni majority that feels oppressed by Assad and his fellow Alawites who dominate the administration has been among causes for concern among those who fear a sectarian civil war similar to that which devastated Iraq over the past decade.
"There's no question that a lot of Syrians fought with al Qaeda elements in Iraq and it's likely that many rebels today learned bombing skills fighting there," said analyst Joseph Holliday, from the U.S.-based Institute for the Study of War.
Armed attacks on military convoys travelling through the countryside have been overshadowed in recent weeks by blasts in Syrian cities targeting security force offices and other symbols of the Assad state, such as the central bank.
But Holliday said it could still not be ruled out that the government was orchestrating at least some of those attacks, especially those which have produced images on state television of bloodied civilians denouncing the rebels as terrorists.
"When rebels talk about making bombs now, most of them are likely referring to their use of explosions for military targets or army convoys," Holliday said. "I think that is different from targeting infrastructure in cities."
"MOTHER OF INVENTION"
Since the army routed them from their strongholds in cities, some rebels said they realized that even in guerrilla street battles they could not beat Assad's tanks or artillery.
The Syrian Liberation Army's spokesman Qdemati said his group's fighters were now focusing most of their attention on "manufacturing facilities" for bombs.
"You are going to start seeing an escalation as we improve our techniques of bomb-making and delivery."
An online statement on Islamist forums from the obscure al-Nusra Front claimed responsibility for a suicide bombing in Damascus on Friday that killed at least nine people. It has claimed responsibility for other suicide bombs in Damascus and one in the northern city of Aleppo.
Rebels say that without more support from foreign states, their struggle is becoming more chaotic and such radicals could play a bigger role.
Last week, Lebanese security forces said a leader from Lebanon's radical Sunni Islamist Fatah al-Islam group died in Syria, apparently planting a bomb.
But many fighters insist that given their meager means makeshift bombs are necessary to fight for a cause that has widespread support in among Sunni Arab states and the West.
Those who have given up on smuggling rifles say the switch has let them channel rare outside donations into better materials that have let them develop more sophisticated bombs.
"We have to be smart about this. Until there is a way to smuggle in anti-aircraft or anti-tank missiles, we won't win with arms," the first Idlib fighter said in Turkey.
"The rebels are getting better at bomb-making; as you know, desperation is the mother of invention."
(Editing by Alastair Macdonald)