The UN has asked the Syrian government to allow international experts "unconditional and unfettered access" to investigate the alleged use of chemical weapons, dpa reported.
The UN high representative for disarmament, Angela Kane, wrote to Damascus asking for the investigation to begin, UN spokesman Martin Nesirky said in New York.
UN Secretary General Ban Ki-moon asked Damascus to respond to the letter "swiftly and favourably so a fact-finding mission can carry out its work in Syria," Nesirky said.
The head of the mission, Ake Sellstrom of Sweden, was due in New York on Monday for talks with UN officials, Nesirky said.