REUTERS - At a desert base, Gulf state Qatar is covertly training moderate Syrian rebels with U.S. help to fight both President Bashar Assad and Islamic State and may include more overtly Islamist insurgent groups, sources close to the matter say.

The camp, south of the capital between Saudi Arabia's border and Al Udeid, the largest U.S. air base in the Middle East, is being used to train the Free Syrian Army (FSA) and other moderate rebels, the sources said.

Reuters could not independently identify the participants in the program or witness activity inside the base, which lies in a military zone guarded by Qatari special forces and marked on signposts as a restricted area.

But Syrian rebel sources said training in Qatar has included rebels affiliated to the "Free Syrian Army" from northern Syria.

The sources said the effort had been running for nearly a year, although it was too small to have a significant impact on the battlefield, and some rebels complained of not being taught advanced techniques.

The training is in line with Qatar's self-image as a champion of Arab Spring uprisings and Doha has made no secret of its hatred of Assad.

Small groups of 12 to 20 fighters are identified in Syria and screened by the Central Intelligence Agency, the sources said.

Once cleared of links with "terrorist" factions, they travel to Turkey and are then flown to Doha and driven to the base.

Ground force

"The U.S. wanted to help the rebels oust Assad but didn't want to be open about their support, so to have rebels trained in Qatar is a good idea, the problem is the scale is too small," said a Western source in Doha.

The CIA declined to comment, as did Qatar's foreign ministry and an FSA spokesman in Turkey.

It is not clear whether the Qatari program is coordinated with a strategy of Western and Gulf countries to turn disparate non-Islamist rebel groups into a force to combat the militants.

Such efforts have been hampered by Western hesitancy about providing significant military aid, because it could end up with extremists. Gulf states dislike the West's emphasis on fighting Islamic State. Assad is the bigger problem, they say.

"Moderate rebels from the FSA and other groups have been flown in to get trained in things like ambush techniques," said a source close to the Qatari government who asked not to be named due to the sensitivity of the topic.

"The training would last a few months, maybe two or three, and then a new group would be flown in, but no lethal weapons were supplied to them," one of the sources said.

Screening process

As the war against Assad has dragged on, frustrated rebels asked their trainers for more advanced techniques, such as building improvised explosive devices (IEDs), requests which were always denied.

"They complain a lot and say that going back they need more weapons or more training in IEDs but that's not something that's given to them," said a Qatar-based defense source.

The Qatar project was conceived before the declaration of the hardline Islamic State, when militants belonging to its predecessor organisation were not regarded as an international security threat.

The group's rise in Syria and Iraq has hampered the rebellion: Moderate groups cannot fight Assad when the better-armed Islamic State seeks their destruction as it strives to build its "caliphate".

In recent weeks, the Qataris, disappointed by lack of progress in the fight against Assad, have started to consider training members of the Islamic Front, a coalition of Islamist rebels less militant than Islamic State or the al Qaeda-linked Nusra Front, but stronger than the FSA.

None have been trained as yet, but Qatar has sought to identify candidates, the sources say.

Some analysts say screening Islamic Front fighters would be harder than FSA rebels, since some Islamists have switched between various groups.

Islamist network

Training fighters from Islamic groups could displease fellow Gulf state the United Arab Emirates, which dislikes Qatar's support for the Muslim Brotherhood's international Islamist network.

But Saudi Arabia, which shares the UAE's mistrust of the Brotherhood, is more indulgent of moderate Islamist forces when it comes to fighting Assad, diplomats say.

Asked about the Qatari training, a Saudi defense source said: "We are not aware of this training camp, but there's one thing we agree on: Assad needs to go and we would not oppose any action taken towards that goal."

To Qatar, ousting Assad remains a priority and youthful Emir Sheikh Tamim has said that military efforts to tackle Islamic State will not work while the Syrian president remains in power.

A source who works with rebel groups said Qatar had delivered weapons, mostly mortar bombs, to the Islamic Front and some FSA brigades about two months ago and had paid some salaries for Islamic Front groups.