The first shipment of chemical weapons materials left Latakia port Tuesday under a deal to rid Syria of its chemical arsenal, the joint mission overseeing the disarmament said.
On the ground, the head of Syrian Al-Qaeda affiliate Al-Nusra Front urged an end to four days of clashes between rebels and the jihadist Islamic State of Iraq and the Levant that has killed at least 274 people.
ISIL responded by calling for all other rebel groups to be "crushed" and also said Syrian opposition politicians were a "legitimate target".
"A first quantity of priority chemical materials was moved from two sites to the port of Latakia for verification and was then loaded onto a Danish commercial vessel today," the UN-Organisation for the Prohibition of Chemical Weapons (UN-OPCW) mission said.
Escorted by Chinese, Danish, Norwegian and Russian naval vessels, the ship will stand offshore until more chemicals arrive at Latakia and then return to collect them.
Tuesday's shipment "initiates the process of transfer of chemical materials from the Syrian Arab Republic to locations outside its territory for destruction," the UN-OPCW statement said.
Syria agreed last year to a US-Russian deal to hand over its chemical weapons.
That came after US President Barack Obama threatened air strikes after an August chemical weapons attack outside Damascus killed hundreds of people, and which Washington blamed on the Syrian regime.
Under the plan, the chemicals will be transported to an Italian port and offloaded onto a US vessel, where they will be destroyed at sea.
The mission has a June deadline to complete the destruction of Syria's chemical weapons, which include the deadly nerve agent sarin and mustard gas.
At OPCW headquarters in the Hague, director general Ahmet Uzumcu called the first removal an "important step".
"I encourage the Syrian government to maintain the momentum to remove the remaining priority chemicals, in a safe and timely manner, so that they can be destroyed... as quickly as possible," a statement said.
Nusra chief urges end to clashes
Meanwhile, clashes raged between fighters from the jihadist ISIL and a coalition of moderate and Islamist rebels.
The Syrian Observatory for Human Rights reported heavy fighting in Raqa, the only provincial capital that has fallen from regime hands and a one-time ISIL stronghold.
The Observatory said 274 people had been killed in the clashes since Friday -- 129 rebels, 46 civilians and 99 ISIL members.
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More than 130,000 people have been killed in the Syrian conflict since March 2011.
The coalition attacked ISIL, angered over its alleged abuses in rebel-held areas, including the kidnapping, torture and killing of civilians and rival rebels.
Nusra Front chief Abu Mohamed al-Jolani, in an audio message posted on Twitter, warned the fighting could give "new life" to President Bashar al-Assad's regime.
"The regime will gain new life when it was close to collapse and the West and the rafidain (Shiites and Alawites) will find a great space," he added.
It "risks costing us dearly on the ground if it continues".
Jolani said ISIL's "flawed policy" had played "a key role in fuelling the conflict", and proposed an initiative to end the fighting.
This would include a ceasefire, a prisoner exchange and setting up an Islamic committee to mediate disputes.
Jolani urged fighters "to give priority to the fight against the regime".
But a defiant ISIL audio message urged its fighters to "crush" other rebel groups.
ISIL Spokesman Abu Mohammed al-Adnani called on the militants to "crush them (the rebels) totally and kill the conspiracy at birth".
He also threatened all members of the opposition Syrian National Coalition.
"Everyone who belongs to this entity is a legitimate target for us, in all places, unless he publicly declares his rejection of that group and of fighting the mujahedeen," Adnani said.
Last April, ISIL chief Abu Bakr al-Baghdadi sought to merge his group and Al-Nusra, which Jolani rejected.
US and Russian-backed peace talks aimed at ending the bloodshed are due to start in Switzerland on January 22.
On Tuesday, Washington accused Iran of aggravating the conflict as it prepared to discuss the potential role of Tehran, a key Assad ally, at talks.
"Iran has done nothing but... help bring foreign fighters in, help the regime's efforts to brutalise the Syrian people," State Department spokeswoman Jen Psaki said.