By Jonathan Saul and Mariam Karouny

LONDON/BEIRUT (Reuters) - Syrian rebels are smuggling small arms into Syria through a network of land and sea routes involving cargo ships and trucks moving through Turkey, Lebanon and Iraq, maritime intelligence and Free Syrian Army (FSA) officers say.

Western and regional powers deny any suggestion they are involved in gun running. Their interest in the sensitive border region lies rather in screening to ensure powerful weapons such as surface to air missiles do not find their way to Islamist or other militants.

FSA fighters say munitions supply chains remain tenuous. In one clash last week, rebel fighters say they ran out of ammunition which forced them to retreat from one of their strongholds in the northern Idlib province.

The steady trickle of relatively unsophisticated arms making its way to forces opposing President Bashar al-Assad is being financed mainly by wealthy individuals in Saudi Arabia and Qatar, a security source said, as well as from expatriate Syrian supporters.

It complements supplies captured from the Syrian army or brought by defectors.

"There are three gateways from where we are getting weapons and they are not secret - Lebanon, Turkey and Iraq," an FSA officer in Syria said. "The weapons entering Turkey are by sea and in containers."

Turkey, which hosts the FSA in the border area and is in the forefront of diplomatic efforts to unseat Assad, says it is not supplying the rebels with weapons, nor allowing passage of arms through its ports or across its territory. However, officials accept a small amount of weaponry is spirited across an almost 900 Km (550 mile) frontier hard to patrol and always prone to smuggling.

The FSA, for its part, would like more powerful arms to attack government armor and defend or extend territory it has, in a change of tactics, begun to seize in recent weeks.

"Of course the weapons we have and we are receiving cannot determine the battle with Assad and finish him. We need more, much more but it helps inflict damage," the FSA officer said.

Several security and maritime intelligence sources said light arms were being procured especially from the Black Sea region.

"Certainly they are being sourced from the Black Sea area and are coming in by ship," a maritime intelligence source said. "We are only talking about semi-automatic weapons which are easily, freely and cheaply obtainable. Each one costs only a few hundred dollars."

A source within the international arms industry said at least one consignment comprising around 2,000 weapons of Russian-pattern Kalashnikov assault rifles and also sniper rifles had been dispatched via Turkey to Syrian rebels in the past few weeks.

"I expect there will be more coming soon from the same places as these arms came from and there are enough (arms) factories around the Black Sea," the source said. "It's small arms and light weapons at the moment."

Arms trackers say Bulgaria, Romania and Ukraine all have stockpiles of Russian-styled light arms that were produced in the countries dating back to the Soviet era when factories were set up with help from Moscow.

"The Syrian army uses primarily Soviet or Warsaw Pact calibers," said James Bevan of Conflict Armament Research, who tracks weapons for governments and organizations.

"And if you look at any rebel force, they tend to use exactly the same weapons as their opponents because when they capture weapons they can use the same ammunition."

A former official with Bulgaria's security sector said the Black Sea was the most realistic route for weapons.

"Because of Bulgaria's gradual decrease of armed forces, the sell-off of arms equipment and the deposition of ammunition, relatively smooth supplies for such deals can be ensured," the former official said.

Bulgaria's foreign ministry said the controls over arms deals were very tight and such shipments were not possible. Sofia, it said, did not export weapons to Syria.

"Our efforts are focused on working with our Arab partners, the EU, the U.S. and Turkey in supporting the opposition in its efforts to develop a credible plan for a free and democratic Syria," said foreign ministry spokeswoman Vesela Cherneva.

Ukraine's security service SBU said Kiev had a strong system of control over weapons sales. Romania's foreign ministry said Bucharest had curbs that would stop deliveries to Syria.

Analysts said despite the controls, Black Sea countries remain viable gateways given the profitability of such trade for private arms dealers and lax scrutiny.

A NATO official said it had "no mandate and no role in intercepting arms consignments of any kind in the Black Sea".

Assad's opponents say just under 13,000 armed and unarmed opponents and around 4,300 members of security forces loyal to Damascus have been killed since he launched a crackdown, using tanks and helicopter gunships to attack rebel strongholds inside Syria's biggest cities.

BY LAND AND BY SEA

Maritime security sources said consignments were most likely to be transported in ships to Turkey for onward dispatch to rebels inside Syria. Ship containers are hard to track and seen throughout the world as a security concern.

Not every port security has scanning facilities to cope with a large turnover of cargo.

The FSA officer said they had also bought assault rifles and grenades from Iraq, which were trucked across the border or carried on donkeys. Equipment also crossed from Lebanon.

Rebels paid about 350,000 Syrian pounds ($4,000) for Katyusha rockets and 100,000 pounds for anti-tank missiles.

"We get LAW (anti-tank missiles) and Katyusha rockets as well as ammunition like bullets and grenades. We buy them from arms dealers - it is easy in Lebanon," the officer said. "The Jordanians have blocked most of our supplies from there."

Rebel fighters have said they were also in possession of thousands of Belgian FN FAL assault rifles. Continued...