* Nusra Front is one of most organised rebel groups
* Front says priority is for unity inside Syria
* Says is not formally allied with Iraqi group
Syria's rebel al-Nusra Front, one of the most effective forces fighting President Bashar al-Assad, pledged allegiance to al Qaeda leader Ayman al-Zawahri in an audio recording posted on the Internet on Wednesday.
Abu Mohammad al-Golani also appeared to distance his group from a recently announced merger with al Qaeda's Iraq branch.
The group's allegiance to Zawahri would trouble Western nations which support the revolt against Assad and will dismay Syrian moderates who want a civil state but fear the rise of extreme Islamist militancy.
"The sons of Nusra Front renew their pledge (of allegiance) to the Sheikh of Jihad Ayman al-Zawahri and declare obedience," Golani said in the recording.
Golani's statement came a day after the leader of the Islamic State of Iraq, Abu Bakr al-Baghdadi, said his group and the Nusra Front would operate under a united leadership called the Islamic State of Iraq and Levant.
Golani said that his group was not consulted before that announcement and he had first heard about it through the media.
Golani said that while the Nusra Front had received assistance from the Islamic State of Iraq, his group would continue operating under its own banner, with loyalty to Zawahri.
"The banner of the Front will remain the same, nothing will change about it even though we are proud of the banner of the (Islamic) State and of those who carry it," he said.
The militant Islamist element of the Syrian conflict poses a quandary for Western powers and their Arab allies, which favour Assad's overthrow but are alarmed at the growing power of Sunni Muslim jihadi fighters whose fiercely anti-Shi'ite ideology has fuelled sectarian tensions in the Middle East.
Golani said that Nusra in Syria will remain "in debt" to the help received from the Iraqi group which he said helped in the formation of Nusra.
Rebels familiar with Nusra's thinking say the group seems to have learned from the experience of its kindred Iraqi group which became powerful in the western province of Anbar after the U.S.-led invasion that toppled Saddam Hussein in 2003.
But its practice of an extreme interpretation of Islam alienated people and residents of Anbar later drove them out.
A Nusra fighter said the group had been surprised by the announcement from the Iraqi wing, which he said had not been coordinated and that an explanation from Baghdadi was awaited.
"Our priority at the moment is unity inside Syria," rather than merging with the Iraqis, he said.
Some Islamists said the main import of the audio statement was not the support for Zawahri, which they said had always been the case, but the response to Baghdadi.
"This statement came just to clarify the statement attributed to al-Baghdadi, God protect him, in which it was announced the formation of the Islamic state, something that did not really happen," a fighter with links to al-Nusra told Reuters.
"Nusra will remain as it is with allegiance to Sheikh Zawahri."