The bus was carrying fighters for Syria's civil war, the observatory reported.
Russian warplanes targeted US, British outpost in Syria: report
Pictures circulating on social media, which Reuters could not immediately verify, showed the burnt out remains of a bus and medics treating injured people.
Turkey's CNN Turk television reported that the explosion took place at the entrance of Atmeh refugee camp in Syria, near the border crossing, citing local sources.
51 civilians killed in Aleppo raids, shelling: monitor
Syria's Idlib province, where Atmeh is located, is a major bastion of the Turkey-backed opposition to President Bashar al-Assad's rule.
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