* Rebel groups cooperate as means of survival
* They say their Western allies have left them in the lurch
* Ideological differences means groups will not merge
By Suleiman Al-Khalidi and Tom Perry
AMMAN/BEIRUT, Sept 29 (Reuters) - A Russian-backed offensive in Aleppo is pushing nationalist rebels to work more closely with jihadists, further complicating a Western policy built around supporting the moderate opposition to President Bashar al-Assad.
As the government mounts a fierce attack on the city, some moderate rebels say the West's failure in Syria has left them with no choice but to coordinate more closely with jihadist groups - the opposite of what U.S. policy has sought to achieve.
In Aleppo, rebels fighting under the Free Syrian Army banner are sharing operational planning with Jaish al-Fatah, an alliance of Islamist groups including Jabhat Fateh al-Sham, the Syrian wing of al Qaeda until it broke off relations in July.
Meanwhile, in nearby Hama province, FSA groups armed with U.S.-made anti-tank missiles are taking part in a major offensive with the al Qaeda-inspired Jund al-Aqsa group that has diverted some of the army's firepower from Aleppo.
This was not the preferred choice of the FSA rebels. They have deep ideological differences with the jihadists, and have even fought them at times. One senior rebel leader said any kind of political merger with jihadists remained out of the question.
But survival is the main consideration as the West struggles to find any way to deter Damascus and its Russian and Iranian backers from pressing a campaign that threatens to snuff out the revolt's most important urban stronghold - eastern Aleppo.
"At a time when we are dying, it is not logical to first check if a group is classified as terrorist or not before cooperating with it," said a senior official in one of the Aleppo-based rebel factions.
"The only option you have is to go in this direction."
Western powers and Assad's regional foes including Turkey and Saudi Arabia have built much of their Syria policy around supporting the FSA rebels, and have given them weapons via coordination centres run by Assad's foreign enemies.
The United States has however remained cautious about the degree of support given to these groups, opposing deliveries of anti-aircraft missiles for fear they could end up in the hands of jihadist groups.
Rebels say their foreign backers have left them hopelessly outgunned by the Russian warplanes and Iranian-backed militias that have tipped the conflict Assad's way in the last year.
Despite deep misgivings, FSA rebel groups supported U.S. efforts to promote a diplomatic solution with Russian cooperation this year. But the rebels say the U.S. approach has resulted in nothing but setbacks.
REBEL CRITICISM OF U.S.-RUSSIAN DEAL
One Aleppo group, Nour Eddin al-Zinki, last week joined an operations room to coordinate military action with the Islamist Jaish al-Fatah alliance. Steps are also being taken for other groups to join, said Abdul Hamid Turki, a senior member of Zinki's political office.
"We joined the operations room of Jaish al Fateh in the area they operate in," Turki said.
Captain Abdul Salam Abdul Razak, the group's military spokesman, said: "The revolutionary brigades cannot do anything against the aggression by a great power and its most potent weapons.
But he said the rebels could take military action to break the siege of rebel-held eastern Aleppo and could also increase coordination with other rebel factions.
Fateh al-Sham, fighting as part of the Islamist Jaish al-Fatah alliance, played a vital role in breaking a government blockade on eastern Aleppo in August, only for the government to besiege it again a few weeks later.
Under the now-shattered U.S.-Russian ceasefire deal, nationalist rebels were to disengage from the jihadist Nusra Front on the western Syrian fronts where they operate in close proximity to each other. Russia and the United States were then to begin joint targeting of both Nusra and Islamic State.
The FSA rebels were scathingly critical of the agreement, attacking what they saw as U.S. double standards that deemed the Islamist Fateh al-Sham a group worthy of attack while failing to mention the Shi'ite Islamist militias that back Assad.
Russia has cited a U.S. failure to get the rebels to disengage from the jihadists as a reason for the agreement's failure. Washington says Moscow and Assad have simply forgone diplomacy in pursuit of military victory.
A LONG-TERM GUERRILLA WAR?
In some respects, the political division between the jihadist and nationalist rebels has recently widened, notably over rebel involvement in a Turkish-backed campaign to secure the northern border from Islamic State and Kurdish groups.
Fateh al-Sham issued a statement effectively prohibiting participation in the campaign, though it is hostile to both Islamic State and the Kurdish YPG group. Its position echoes rebel criticism that the Turkish-backed campaign has been a distraction from the rebels' war with Assad.
Noah Bonsey, senior analyst at the International Crisis Group, said he did not expect the FSA and jihadist groups to merge, due to political and ideological divisions and the conditions attached to the state support to the rebels.
But the government's Aleppo offensive would "clearly push all non-Daesh (Islamic State) rebel groups towards more cooperation", he said.
"There is little doubt that Fateh al-Sham's efforts to convince others to join with it are likely to gain appeal if pro-regime offensives continue at this scale and the opposition's state backers are unable to provide alternative means of addressing rebel defence requirements", he said.
U.S. officials said on Wednesday that the Obama administration has begun considering tougher responses to the Russian-backed Syrian government assault on Aleppo, including military options. But it was not clear what, if anything, the president would do.
Rebel groups are meanwhile reviewing their strategy.
The senior official in an Aleppo-based faction said forgoing territory may be one element of a new military approach by the rebels. "We are in a liberation war against Russia and Iran," he said.
"The coming months or even days will witness a change in military strategy," he added. "It could turn into a long-term guerrilla war."
(Writing by Tom Perry; editing by Giles Elgood)
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