Russia has said it had told Syria to cooperate with UN experts investigating reports of a deadly chemical weapons attack that is believed to have killed hundreds, and called on rebels to allow access to the area.

Foreign Minister Sergei Lavrov told his US counterpart John Kerry in a phone call that immediately after the reports first emerged about the attack on Wednesday, the "Russian side called on the Syrian government to cooperate with the UN chemical experts," the foreign ministry said in a statement on Friday.

Al Jazeera's Charles Stratford reports.