Russia's military on Friday insisted no aircraft flew over a Syrian refugee camp the day it was hit by deadly strikes, suggesting the al-Qaida-linked Al-Nusra Front could have shelled the location.

"We have attentively studied the information from the air space monitoring data in this area for May 4 and 5, 2016. There were no flights by Russian or any other aircraft," spokesman Igor Konashenkov told Russian news agencies.

"Judging by the damage shown in photographs and video, the camp may have been shelled either on purpose or by mistake by multiple rocket launchers which are currently being used very actively in this area by terrorists from Al-Nusra," Konashenkov said.

Syria's military has denied any involvement in the Thursday raids that killed at least 28 civilians in territory controlled by Syria's al-Qaida affiliate Al-Nusra and rebel allies.

The United Nations' rights chief said Friday that air strikes on a camp for displaced people in Syria's Idlib province were almost certainly not accidental and likely amounted to a war crime.

Rights chief Zeid Ra’ad Al Hussein said responsibility for the strikes remained unverified but cited "initial reports" indicating that Syrian President Bashar al-Assad's forces were to blame.

Women and children were reported to be among the civilians killed in the strikes close to the Turkish border, which left 50 others wounded.