* Leadership to be based in Idlib or Aleppo
* Rebel-controlled Syria still under aerial attack
* Colonel says FSA will fight with other rebel groups
* Syria internal opposition to hold Damascus conference
BEIRUT, Sept 22 (Reuters) - The rebel Free Syrian Army (FSA) has moved its leadership for the first time from Turkey to parts of Syria that are now controlled by rebels, the group's commander-in-chief said on Saturday.
The FSA has been based in Turkey for more than a year as fighters have struggled to battle forces loyal to President Bashar al-Assad. Although rebels now control large swathes of Syria, they face air and artillery attack from Assad's forces.
"The leadership of the FSA has entered the liberated areas (of Syria) after the success of the plan that the FSA has worked on with other battalions and units in order to safeguard the free areas," Colonel Riad al-Asaad said in a video statement.
A rebel source close to Asaad said that the colonel arrived in Syria two days ago. "The plan is that all the leadership of the FSA will be based in Syria soon, either in Idlib province or Aleppo province," the source told Reuters, adding that the move would be completed within two weeks.
The rebels made their announcement on the eve of a conference of several government-sanctioned Syrian opposition groups in the capital Damascus aiming to provide a political solution to the civil war - a meeting which the FSA dismissed as a ploy by Assad to fool the international community.
The FSA is the most prominent of several armed groups fighting to overthrow Assad. In the video, posted on the web, the rebel colonel said his men would "fight side-by-side" with all groups and planned to take Damascus soon.
Despite calling for Assad to step down, the West is wary of arming disparate rebel groups. Western diplomats say they are looking for signs that the rebels have a clear chain of command within Syria.
Turkey, which is housing more than 80,000 refugees from Syria, is facing internal pressure to distance itself from the conflict, and rebels are not always welcomed by residents.
Rebels shot down a fighter jet as it flew over the northern Syrian town of Atarib in Idlib province on Saturday, a witness said.
The witness, an independent journalist who asked to remain anonymous, said rebel fighters were attacking a military base near the town when the jet flew over and rebels shot it down with anti-aircraft guns.
Rebels have previously brought down several government planes using outdated anti-aircraft machine guns welded to pickup trucks.
Activists say more than 27,000 people, mostly civilians, have been killed in the 18-month-old revolt in Syria.
In Damascus, the army has been conducting raids of southern suburbs over the past few days after driving out most rebel fighters. Black smoke rose from the suburb of Hajar al-Aswad on Saturday and residents said their houses had been set ablaze by security forces.
The opposition conference in Damascus on Sunday is organised by the internal opposition's main umbrella group, the National Coordination Body.
Last July, a similar conference was cancelled after the owner of the venue was threatened by Assad's forces who fired on a pro-democracy protest outside, killing 14.
Opposition groups say Russia and China, which have blocked Western attempts to secure U.N. sanctions against Assad, have promised to exert influence to protect Sunday's meeting.
Assad says he accepts some opposition figures who call for a peaceful transition from a one-party state to democratic governance and his allies have pointed to the internal opposition as a sign the president is serious about reform.
The internal opposition, which includes many outspoken critics of Assad who have spent years in jail, has been accused of being too passive by rebel fighters and members of the largely foreign-based Syrian National Council, a political group calling for the international community to arm rebel factions.
A spokesman for the Free Syrian Army in Turkey dismissed the conference, saying Assad's government "tries always to negotiate with itself".
"This is not a real opposition in Syria. This opposition is just the other face of the same coin. The FSA would never have any relation with these groups," he told Reuters by telephone.
"It is just a silly plot to mislead the international community to think there is a negotiation in place. They cannot be successful in finding an end to the civil war."