The head of Arab TV channel Al Jazeera said on Tuesday he was leaving the network, but gave no reason for his departure at a time when the station's coverage has played an important role in unprecedented protest movements rocking the Arab world.

I have decided to move on, the network's director-general Waddah Khanfar said in a resignation note emailed to Al Jazeera staff and also publicised on social media site Twitter.

For some time I have been discussing my desire to step down with the chairman of the board. He has kindly expressed understanding and has accepted my decision.

Sheikh Ahmad bin Jasem al-Thani, a member of Qatar's ruling family who is a member of a top body at state-run Qatargas, was named to replace Khanfar, an Al Jazeera statement said.

Upon my appointment I set a goal to establish Al Jazeera as global media leader. This target has been met and the organization is in a healthy position, Khanfar said in a statement issued by the station.

Since it was launched in 1996, Al Jazeera has become the highest-profile satellite news broadcaster in the Middle East. It frequently has had difficulties with Western and Arab governments in a region where governments have traditionally kept tight control over state media.

Al Jazeera, owned by the Qatari government, aired round-the-clock coverage of uprisings that brought down veteran rulers in Tunisia, Egypt and Libya this year, and the station promotes itself as a democratic voice in the region.

Critics say it is more timid in covering events closer to its Gulf home, and the cameras of its main Arabic channel were notably absent during a month of similar protests in the Gulf Arab state of Bahrain which the government crushed in mid-March.

Al Jazeera's bureau chief in Lebanon, Ghassan Bin Jiddo, resigned in April, apparently in disagreement over its coverage of the revolts, which have also engulfed Syria and Yemen.

Leaked U.S. diplomatic cables described the channel as a tool in Qatari diplomacy. The channel has played an important role in raising the prestige of the small, wealthy Gulf Arab state.

(Reporting by Nour Merza and Firouz Sedarat; Editing by Andrew Hammond and Michael Roddy)

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