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 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.
FSA leader Colonel Riad al-Asaad said in a video statement that "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".
The rebels made their announcement on the eve of a conference of several government-sanctioned Syrian opposition groups.
Their aim is 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.
Elsewhere, Rebel fighters trying to oust President Bashar al-Assad shot down a fighter jet as it flew over the northern Syrian town of Atarib in Idlib province, 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.
Vastly outgunned, rebels say they need surface-to-air missiles to take down planes and helicopters used by the Syrian military to bombard opposition strongholds.
Fighters use outdated anti-aircraft machine guns that are welded to pickup trucks but they are inaccurate and useless if the military aircraft fly above a certain altitude.
On 27 August fighters shot down a helicopter on the outskirts of Damascus and three days later rebels said they had brought down a jet in Idlib, near the Turkish border.
Activists say more than 27,000 people, mostly civilians, have been killed in the 18-month-old revolt in Syria, which began with peaceful street protests that provoked a military crackdown and mushroomed into civil war.