NEW YORK, Oct. 8 (UPI) — Despite efforts by the United Nations to identify all of Syria’s chemical weapons facilities, the Syrian government reportedly failed to disclose four additional such facilities.

U.S. Ambassador to the U.N. Samantha Power voiced her concern about this revelation Tuesday following a U.N. Security Council meeting with OPCW-U.N. Joint Mission Special Coordinator Sigrid Kaag.

In her briefing, “Sigrid Kaag said 4 facilities identified that regime failed to declare. Must keep pressure on regime so it doesn’t hide CW capability,” Power wrote on Twitter,

According to an unidentified Western diplomat who spoke with CNN about Kaag’s testimony, the sites include three research and development facilities and one production facility.

Power earlier raised the issue in September about “discrepancies and omissions related to the original Syrian declaration,” particularly because of U.S. concern about possible seizure and use by extremist terrorist groups or continued chemical weapons attacks by the Bashar al-Assad regime.

The OPCW-U.N. Joint Mission completed its mission on Sept. 30. The OPCW announced on Oct. 1 that “The OPCW mission in Syria will continue to deal with the destruction of chemical weapon production facilities and clarification of certain aspects of the Syrian initial declaration.”