The Arab League and Turkey are pressing the Syrian opposition to unite before Western and Arab foreign minister meet in Istanbul next week to back the revolt against President Bashar al-Assad, opposition sources said on Sunday.

Qatar, the rotating chairman of the Arab League, and Turkey, a Muslim member of NATO bordering Syria, asked Assad's divided opponents to gather in Istanbul on Monday and Tuesday ahead of the "Friends of Syria" meeting in the same city on April 1.

The Qatari-Turkish invitation said the aim was to discuss visions for a free and democratic Syria and agree on a set of shared principles for a peaceful political transition, unifying around the Syrian National Council (SNC).

Some liberals and independent Islamists opposed to a growing influence of the Muslim Brotherhood have quit the SNC, an umbrella organisation set up last year, raising doubts about whether opposition to Assad is enough to weld a common front.

"We expect the SNC to be recognized as the only legitimate representative of Syria," Muslim Brotherhood official Faruq Tayfur told a news conference in Istanbul. [Reuters]