By Erika Solomon

ALEPPO, Syria (Reuters) - Abu Bakr, a Syrian rebel commander on the outskirts of Aleppo, is a devoted Islamist determined to overthrow President Bashar al-Assad. But the radical allies that have joined the rebels in recent months alarm even him.

"Let me be clear. I am an Islamist, my fighters are Islamists. But there is more than one type of Islamist," he told Reuters. "These men coming fought in insurgencies like Iraq. They are too extreme, they want to blow up any symbol of the state, even schools."

Seventeen months into the uprising against Assad, Syria's rebels are grateful for the support of Islamist fighters from around the region. They bring weapons, money, expertise and determination to the fight.

But some worry that when the battle against Assad is over they may discover their allies - including fighters from the Gulf, Libya, Eastern Europe or as far as the Pakistan-Afghanistan border area - have different aims than most Syrians.

"Our goal is to make a new future, not destroy everything," Abu Bakr said, sighing as he rattled his prayer beads. "As bloody as it is now, this stage is simple. We all have the same cause: topple the regime. When Bashar falls, we may find a new battlefront against our former allies."

Abu Bakr and his comrades say they envision Syria as a conservative version of Turkey's moderate Islamist rule, not an autocratic theocracy. They are unnerved by a recent kidnapping of foreign journalists and attacks on state infrastructure.

Western powers have warily watched the signs of an increasing presence of foreign Sunni Islamist fighters in Syria.

They fear a repeat of the mass sectarian slaughter that followed the American invasion of Iraq. Sunni Islamist suicide bombers affiliated with al Qaeda there are still targeting security forces and Shi'ites in large-scale bomb attacks.

Some fighters who have come to Syria are idealists who believe in jihad, or holy war, for oppressed Muslims, and would probably return home in a post-Assad era. But others are al Qaeda-linked fighters who may want a base in Syria.

Their numbers are still low, but enough to worry countries fearing Iraq-style bloodshed in Syria, a country straddling the lines of most ethnic and regional conflicts in the Middle East.

LOOK FOR THE BOMBS

Abu Bakr, a short man with a long black moustache, says right now there is no choice but to allow foreign fighters. On a summer night, he and his small daughter waved off a truck crammed with rebels heading into Aleppo.

The fighters have brought in rocket propelled grenades and boxes of homemade explosives. And wherever you find improvised bombs, you're likely to find foreign fighters, says a rebel called Mohammed in another local unit.

"They brought a lot of bomb making experience from the insurgency in Iraq. With their help, our bombs have 3-7 kilometer detonation range. Now, we can even set them off by mobile phone," said Mohammed, who still walks with a slight limp from a freshly healed wound.

He was shot when his unit planted bombs near an air force base. Like other fighters interviewed by Reuters, he denied that he had worked with radicals from abroad.

In some Aleppo neighborhoods hit by heavy army shelling over the past week, there were signs that foreign fighters appeared to be present among rebels.

Some men crouching among gutted buildings wore shalwar kameez, the loose trousers and shirts worn in Afghanistan and Pakistan but uncommon in Syria. They had long beards cleanly cut along their jaw line, a style associated with Salafism, an austere Sunni school which seeks to replicate life in the age of the Prophet Mohamed. As soon as journalists approached, the men vanished into buildings.

Not all rebel groups work with foreigners, and not all Syrian rebels work well with each other. In Aleppo for example, the largest group is the 2,000 strong Tawheed Brigade. It says it accepts foreign fighters, but only if they play by its rules.

"There are some really extremist battalions that don't cooperate well with us. They stay on their own," said a fighter from the Tawheed brigade.

"We're trying to fold jihadis into our group so they back off their more aggressive tactics. That doesn't mean we aren't nervous. They could still turn and rebel against us," he said.

WEAK UNIONS

The Tawheed brigade's leaders, none of whom were military officers, are trying to keep the battle in Aleppo more organized than previous campaigns. Commander Abdulqader Salheen says they aim to divide the city into nine administrative districts and set up leaders for each area to streamline communication.

But there are several other brigades and dozens of independent battalions working independently, and fights are common. The Tawheed brigade's advances in unifying the three-week-old battle for Aleppo began to fall apart when smaller groups complained they were not getting a fair share of the weapons spoils from ransacked police stations.

Some units have even withdrawn back to the countryside over disputes. Tempers are short and everyone has lost siblings, cousins and friends. Most fighters are young, anywhere from 15 to 28 years old, and they are grappling with one of the bloodiest conflicts in the region. Confusion reigns.

At an abandoned military site held by rebels in Aleppo, one young fighter with a scuffed up kalashnikov drew a blank when asked what unit he was in. He consulted a comrade, who told him they were part of a newly formed "Victory Battalion".

One of the most effective and elusive groups in Aleppo now sending reinforcements into Damascus is called Ahrar al-Sham, "The Free Men of Syria." Its fighters accept the bulk of jihadist foreign fighters in Idlib and Aleppo, rebels say.

"They're extremely effective and secretive. They coordinate with us to attack the regime but they don't take orders from anyone. They get weapons and explosives smuggled from abroad that are much better," said a rebel in Aleppo called Anwar.

Other groups are amateurs working alone, and it shows.

When the army fires its helicopter gunships and mortars on them, they crouch in an alley while dust and concrete rains down on them. They peek over their rifles or grenade launchers, and fire randomly.

GULF CASH

It's little surprise then that battle-hardened fighters from abroad, with wads of cash from the Gulf, appeal to rebels. One opposition activist said that groups like Ahrar al-Sham get money from Gulf Islamists in Kuwait and Saudi Arabia. Continued...