Turkey is putting pressure on Syrian opposition groups to unite as Istanbul gets ready to host the key international meeting of Friends of Syria.

About 200 representatives from Syria's political groups, including President Bashar al-Assad's ethnic Alawite group, are taking part in a two-day session in Turkey from 1 April.

The only group left out is the Kurdish Democratic Union Party, which has been banned because its affiliation to the Kurdistan Workers' Party (PKK) puts it on the terrorist blacklist of Turkey, the European Union and the US.

Ankara is pushing ethnic and religious groups to come up with a pact to solve the Syrian impasse before the talks.

"If they agree on a document that puts forth a constitutional vision that endorses everybody ahead of the meeting of the Friends of Syria, then the Syrian National Council [could be recognised] as the sole legitimate representative of Syrian people," an unnamed official told Hurriyet Daily News.

Turkey announced that it has temporarily closed its embassy in Damascus, citing deteriorating security conditions as the main reason for suspending all activities.

Friends of Syria will meet to discuss further action against Assad's regime. The United States plus Arab and European countries are expected to find a solution to the bloody year-long crackdown on civilians by the entrenched regime of President Bashar al-Assad.

The moves to resolve the violence comes as the Free Syrian Army has been accused of using children as fighters.