Despite the destruction of much of Syria's chemical arsenal aboard a U.S. ship in July, the U.N said on Thursday that there seemed to be gaps in Syria's declaration of its chemical weapons.

Sigrid Kaag, head of the Organization for the Prohibition of Chemical Weapons-United Nations (OPCW-UN) joint mission on eliminating Syria's chemical weapons program, said that since the Syrian government declared its chemical arsenal, it has already made four amendments.

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All of the worst toxins that were declared have already been destroyed, while 96 percent of the chemical weapon stock was wiped out, according to Kaag, saying that once the chemical weapon destruction is over, the OPCW will continue its support of U.N. operations.

"Even with Syria's declaration, there are still some discrepancies or questions that are being asked," Kaag said after briefing the U.N. security council.

According to Kaag, members of the U.N. are concerned about gaps in the declared volume of Syria's chemical weapons, adding that she will be going back to Damascus to discuss the matter.

However, with the Islamic State's advance in large regions of Iraq in Syria, the United States expressed its fear that any chemical weapon that were omitted may fall into the hands of the insurgents.

U.S. Ambassador to the United Nations Samantha Power said if Syria had any chemical arsenal left, there is a great danger of these falling into the hands of the rebels.

"We can only imagine what a group like that would do if in possession of such a weapon," she said.

There are still 12 chemical weapon production facilities that remain to be destroyed, including five tunnels and seven hangars, Kaag said.

In 2013, Syria agreed to the destruction of its chemical arsenal when it joined the Chemical Weapons Convention (CWC).

TagsISIS, UN