By Mariam Karouny

BEIRUT (Reuters) - Syrian rebels have received advanced weapons aimed at narrowing the arms gap with President Bashar al-Assad's forces and reinforcing a new rebel military command which Western countries hope can dilute the strength of Islamist fighters.

Several rebel commanders and fighters told Reuters that a shipment which reached Syria via Turkey last month comprised shoulder-held and other mobile equipment including anti-aircraft and armor-piercing weapons, mortars and rocket launchers.

Rebels told Reuters the weapons, along with money for cash payments for fighters, were being distributed through a new command structure, part of a plan by foreign backers to centralize control over rebel units and check Islamists linked to al-Qaeda. However, in a sign of the difficulty in uniting disparate fighting groups, some rebels said they had turned down the arms and refused to submit to the new command.

While not nearly enough to tip the military balance against Assad, who is able to deploy air power, missiles and artillery to devastating effect against rebel areas, any significant arms shipment is a boost to rebels who have long complained about the lack of international support.

The rebels refused to specify who supplied the new weapons, saying they did not want to embarrass foreign supporters, but said they had arrived openly via Turkey "from donor countries".

"We have received this shipment legally and normally. It was not delivered through smuggling routes but formally through Bab al-Hawa crossing," said a rebel commander in Homs province, referring to a rebel-held crossing with Turkey.

"But it is not enough to help us win," he told Reuters by Skype. "Another shipment has arrived in Turkey but we haven't received it yet," he added, saying he believed foreign donors were waiting for the Syrian opposition to form a transitional government to work with the rebel command.

The political opposition will meet in Istanbul on Saturday to choose a prime minister in the transitional government, which is also supposed to choose a civilian defense minister - creating the basic structure for a future state and army.

The Syrian revolt erupted nearly two years ago, starting with peaceful protests for reform but developing into an armed insurgency and then civil war as Assad responded to the uprising with ever-growing force. The United Nations estimates that 70,000 people have been killed in the relentless violence.

Although many countries backed Assad's opponents, few have actively supported arming the rebels, fearing that weapons might end up in the hands of hardline Sunni Muslim militants and lead to a repeat of Western conflicts, such as the wars against the Taliban in Afghanistan and al Qaeda-affiliated groups in Iraq.

So far rebels have relied mainly on light weapons smuggled from neighboring countries, many of them financed or sent from sympathizers in Gulf states, and from supplies seized from captured army bases inside Syria.

But video footage and pictures from across the country appear to support assertions that advanced weapons - with origins as varied as the former Yugoslavia and China - have ended up in rebel hands.

A Reuters photographer in Damascus over the last month saw several Western-built rebel firearms- including U.S. pattern M4 and Austrian Steyr assault rifles - that almost certainly came from outside the country.

STRENGTHENING REBEL COMMAND

Assad's strongest regional supporter has been Shi'ite Muslim Iran, while the leading campaigners for arming the rebels are the Sunni Muslim Gulf Arab powers Qatar and Saudi Arabia, reflecting the strong sectarian currents of the Syrian uprising.

Although Saudi Arabia and Qatar do not discuss specific weapons shipments to the rebels, both countries have been open about their support for arming them in principle.

Saudi Foreign Minister Saud al-Faisal bluntly told a news conference in Riyadh on February 12: "My country believes that the brutality of the Syrian regime against its own people requires empowering the people to defend itself."

Qatari Prime Minister Sheikh Hamad bin Jassim bin Jabr al-Thani said last week: "As there is no clear international opinion to end the crisis in Syria...we are supporting the opposition with whatever it needs, even if it takes up arms for self-defense."

Western countries have been more cautious, and have so far committed publicly to sending only "non-lethal" aid, like radios and body armor.

International powers are alarmed by the growing influence of Islamist hardliners in a country which lies at the crossroads of the Middle East between Iraq, Israel, Turkey, Lebanon and Jordan. They have made efforts to unite Syrian rebels under a clear leadership. A body was formed in December to bring the rebel units, or brigades, together under a unified command.

"One of the reasons for the change in the donors' minds is that they want to empower the new military command. They want to help it organize the weapons and the fighters," said an aide to a rebel commander in a province which has seen some of the heaviest fighting.

"If the brigades join then they get their share of these weapons and also monthly payment for the fighters."

The new military command divides Syria into five fronts - southern, western, eastern, northern and central.

"Each front has received its share. All equally distributed," the rebel said, adding that 'payment' for the weapons would come in the form of post-conflict reconstruction contracts in Syria awarded to countries that helped.

"So basically it's like we have paid in advance. It is funded by the countries that will be involved in reconstruction of Syria," he said.

But in a sign of the continued divisions among Assad's foes, some rebels complain that the "military councils" who received the weapons - and are seen by the West as more likely allies than the hardline Islamists - were the wrong groups to arm. Continued...