The United States expressed concern Thursday that undeclared Syrian chemical weapons could fall into the hands of Islamic extremists.

U.S. Ambassador Samantha Power raised the concerns as a joint U.N. and Organization for the Prohibition of Chemical Weapons mission winds up an drive to eliminate Syria's chemical arsenal.

The mission is supposed to formally conclude its work by September 30, but its chief Sigrid Kaag said there were still "discrepancies and questions" related to Syria's weapons declarations.

"The international community must continue to press for a resolution of all discrepancies and omissions," said Power.

"The U.S. is concerned about all discrepancies and also the potential that there are real omissions in the declaration," she added.

Power said the Security Council, currently presided by the United States, "intends to stay very much on top of this."

"Extremist groups have terrorized everyone they come into contact with in Syria and Iraq and these weapons stocks, if there are any left, could fall into their hands," she said.

The United States has launched an air campaign against Islamic State militants as they have swept through large parts of Iraq from strongholds in Syria.

Power also recalled that the Syrian regime had carried out chemical weapons attacks itself, particularly on August 21 2013 in a Damascus suburb.

Kaag, delivering her last report to the Security Council as the mission's chief, said Damascus had presented "four amendments" to its weapons declaration, and that there had been "constructive" discussions on certain discrepancies, including the volume of toxic chemicals declared.

A total of 1,300 tons of chemical agents have been removed from Syria and subsequently destroyed at sea.