Syria is unlikely to meet a December 31 deadline to move its most dangerous chemical weapons out of the country, the United Nations has acknowledged.

The UN and the Organisation for the Prohibition of Chemical Weapons (OPCW) on Saturday said "important progress" had been made towards eliminating Syria's banned weapons, but urged President Bashar al-Assad's government to "intensify efforts" to meet internationally-set targets.

"Preparations continue in readiness for the transport of most of the critical chemical material from the Syrian Arab Republic for outside destruction," noted a joint UN-OPCW statement. "However, at this stage, transportation of the most critical chemical material before 31 December is unlikely."

The year-end deadline was the first major milestone under a UN Security Council-backed deal arranged by Russia and the United States, which aims to wipe out all of Syria's chemical arms by the middle of 2014.

The UN and OPCW attributed the delay to Syria's worsening civil war, logistical problems and bad weather.

The chemicals will ultimately 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, diplomats say.

Message to the pope

Meanwhile, Assad sent a message to Pope Francis on Saturday expressing his determination to defend Syrians of all religions against hardline Islamist rebels.

The message was passed on through a Syrian government delegation that held talks with Vatican officials.

The official Syrian Arab News Agency said Assad expressed his government's "determination to exercise its right to defend all its citizens, whatever their religion, against the crimes committed by the takfiri (Sunni Muslim extremist) bands who attack them in their homes, in their places of worship and in their neighbourhoods."

Assad's regime prides itself on its secularism, and while the rebels fighting for its overthrow are mainly Sunni, the government draws much of its support from Assad's own Alawite minority, Christians and other minorities.

Earlier this week, the pope pleaded for humanitarian aid access and an end to the violence in Syria, noting: "Too many lives have been shattered in recent times by the conflict in Syria, fuelling hatred and vengeance."