The removal and destruction of the most dangerous agents in Syria's chemical arsenal will likely be delayed until the end of June because of logistical and security problems, the head of the world's chemical weapons watchdog said on Thursday.

Mustard gas and the components for making Sarin and VX, known as "primary" agents, were originally to have been destroyed by the end of March.

Syria has already missed a December 31 goal to transport the most toxic substances to a port and so far has loaded only a relatively small amount of chemicals onto the Danish cargo ship Ark Futura.

Ahmet Uzumcu, head of the Organisation for the Prohibition of Chemical Weapons (OPCW), said he was "confident" that all the chemicals could be destroyed by the end of June - the original deadline for the less dangerous "commodity" chemicals.

"As we were not able to meet the timeline for the 31st of December, from my point of view what is important is really the end of June 2014, so we will do our best to meet it," he said.

The OPCW is overseeing the destruction of the Syrian arsenal as part of an international accord brokered by Russia after poison gas attacks on the outskirts of Damascus killed hundreds, including children, in August last year.

Chemical weapons were likely used in five out of seven attacks investigated by UN experts in Syria.

Uzumcu said only about 16 tons of the total 560 tons of the "primary" chemicals had so far been shifted to the Danish vessel. Once the Danish ship has loaded all the primary agents, it will take them to the port of Gioia Tauro in southern Italy, where they will be transferred to a US ship and later destroyed at sea.

Separately, Russia denied Thursday having a "hidden agenda" on Syria as it launched a fresh round of crisis diplomacy by hosting the top diplomats of Iran and Damascus ahead next week's historic peace talks.

Iranian Foreign Minister Mohammad Javad Zarif arrived in the Russian capital from Damascus on the same jet as Syrian Foreign Minister Walid Muallem.

Zarif met Russian Foreign Minister Sergei Lavrov before the three diplomats were due to try to come up with a joint stance that would keep President Bashar al-Assad in power when the Syrian peace talks begin in Switzerland on Wednesday. "This does not mean that we have some tri-party [peace] draft," Lavrov told reporters at a joint press with Zarif.

"We have nothing to hide," said Lavrov. "We have no hidden agenda."

Reuters - AFP