The U.N.'s top Mideast envoy says there are mounting reports of chemical weapons use in Syria.

In response, Robert Serry says the United Nations is again urging the Syrian government to allow chemical weapons experts into the country without further delay to investigate the allegations.

Syria asked Secretary-General Ban Ki-moon to investigate an alleged chemical weapons attack by rebels on March 19 on Khan al-Assal village in northern Aleppo province and wants a probe limited to that incident. But Ban is insisting on a broader investigation, including a December incident in Homs raised by Britain and France.

Serry, the U.N. special coordinator for the Middle East peace process, told the U.N. Security Council Wednesday that Ban remains "gravely concerned" about the allegations of chemical weapons use.