The June deadline to destroy Syria's chemical arms can still be met despite delays moving the most dangerous chemicals, the head of the UN mission monitoring the operation said Wednesday.

Sigrid Kaag, head of the UN-Organization for the Prohibition of Chemical Weapons joint mission, expressed guarded optimism after the first chemicals were loaded onto a ship in the Syrian port of Latakia on Tuesday.

Syria's most dangerous chemicals were meant to have been moved out of the country by December 31. Under a UN-backed plan, all of Syria's declared 1,290 tonne arsenal should be destroyed by June 30. But the country's worsening conflict has caused holdups.

"I did not say we are on schedule," Kaag told reporters after briefing the 15-nation UN Security Council on the operation.

She said there was a "collective expectation" by the Security Council "that looking at the end of June deadline that there is no reason to assume that delays should occur. All things being equal."

"Everything is ready, investment is made and the authorities have shown that first movements have started to happen," she added, describing Tuesday's load as "an important first step."

But Kaag stressed that security changes each day hold up the unprecedented international operation ordered after a chemical weapons attack near Damascus in August in which hundreds died.

"Security when it comes to access to sites, getting caught in crossfire, direct fire, indirect fire, is a concern for the joint mission," she said.

On top of battles between President Bashar al-Assad's forces and opposition rebels, a customs strike in neighboring Lebanon and heavy snow in Syria had blocked the delivery of necessary equipment, Kaag said.

Containers of Class A chemicals from Syria's arsenal were put on a Danish vessel in Latakia on Tuesday which is now being guarded at sea by an international fleet.

After more chemicals have been loaded, the consignment will be taken to Italy to be transferred to a US Navy vessel for destruction to start.

The chemicals "will stay on the ship. They are safe and secure, they are properly guarded," Kaag said of the current operation.

Kaag said an international tender for companies to destroy lower level chemicals in the arsenal would be completed within weeks.

Britain is among countries whose companies have made bids in the tender.