WASHINGTON - Syria's most dangerous chemical weapons will almost certainly not be removed on schedule, according to the international organizations responsible for the operation.

Bashar al-Assad's Syrian regime is cooperating with a joint United Nations/Organization for the Prohibition of Chemical Weapons (OPCW) team to remove chemical weapons for destruction at sea, and the OPCW laid out a Dec. 31 target date when it announced a plan for Syrian CW destruction on Nov. 15.

Citing "continuing volatility in overall security conditions, which have constrained planned movements," the joint UN/OPCW mission announced today that "at this stage, transportation of the most critical chemical material before 31 December is unlikely. … Logistical challenges coupled with inclement weather have contributed to this delay."

Syria's chemical weapons must be transported through some contested territory before they can be shipped out for destruction at sea aboard the U.S. ship MV Cape Ray in an international operation. Assad's regime is providing security for the UN/OPCW team and the removal of the weapons.

The Dec. 31 target had appeared dubious of late. A top Russian diplomat said publicly this week that it probably wouldn't be met, and the State Deptartment responded by calling the date a "milestone," not a deadline, in an unworried-sounding statement.

While on-the-ground security challenges, weather, and a weighty procurement process for equipment have contributed to the delay, and while the UN/OPCW mission referenced Syrian cooperation, it nonetheless called on Syria to do more.

"It remains the ultimate responsibility of the Syrian Arab Republic to ensure the safe packaging, transport, and removal of chemical weapons material and to facilitate the procedures for the expeditious and verifiable destruction of its remaining chemical weapons," the UN/OPCW mission said. "The Syrian Arab Republic needs to intensify its efforts to ensure that its international obligations and commitment are met."

The OPCW Executive Council will meet and report to the UN Security Council on Jan. 8. It is unclear whether Syria's chemical weapons are expected to be moved to port by then.