The UN and the Organization for the Prohibition of Chemical Weapons (OPCW) said "important progress" has been made on eliminating Syria's banned weapons, but called on President Bashar al-Assad's government to "intensify efforts" to meet internationally set deadlines.
The year-end deadline was the first key milestone under a UN Security Council-backed deal arranged by Russia and the United States that aims to wipe out all of Syria's chemical arms by the middle of 2014.
"Preparations continue in readiness for the transport of most of the critical chemical material from the Syrian Arab Republic for outside destruction. However, at this stage, transportation of the most critical chemical material before 31 December is unlikely," said a joint UN-OPCW statement.
Syria's worsening civil war, logistical problems and bad weather had held up the operation to move chemical agents to the port of Latakia, the two bodies said.
Under an internationally agreed plan, the chemicals will be taken to a port in Italy where they are to be transported to a US Navy ship specially fitted with equipment to destroy the weapons at sea, according to the diplomats.
Fighting between Assad's forces and opposition rebels has held up transportation of the chemicals, and some details of the destruction operation have still not been finalized, UN diplomats said.
The US-Russia deal for Syria to surrender more than 1,000 tonnes of chemical agents averted US-led military strikes after a chemical weapons attack on August 21 near Damascus that the United States says killed 1,400 people.
The UN and OPCW are monitoring and helping with the operation but the Syrian government has prime responsibility for moving the chemicals.
"Since the Syrian Arab Republic disclosed its chemical weapons program three months ago, important progress has been made," said the UN-OPCW statement.
Syria has started the destruction of equipment at facilities it declared and completed the eradication of missiles intended for chemical weapons use ahead of schedule, said the statement.
The UN and OPCW welcomed "important milestones" already met by Assad's government, but highlighted "the importance of maintaining positive momentum."
They said the Syrian government "needs to intensify its efforts to ensure that its international obligations and commitment are met" under the Chemical Weapons Convention and the Security Council resolution which ordered the destruction of its weapons.
UN leader Ban Ki-moon played down the delay in the weapons destruction insisting in a UN statement that the operation was making "effective progress" as shown by "the steady achievements in meeting all previous milestones the past three months."
"Despite this delay the joint mission continues to work closely and intensively" with the Syrian government and other countries working on the destruction, added the statement released by UN spokesman Martin Nesirky.
The OPCW executive council is to meet again on January 8 to discuss Syria. OPCW director general Ahmet Uzumcu said in a statement that "virtually all of the necessary logistical and security related assets are now available" to start transporting Syria's chemicals.