UN chemical weapons experts have delayed their departure to Syria because of differences with Damascus over how they will investigate alleged attacks, UN diplomats said Monday.

The experts, led by Ake Sellstrom of Sweden, were supposed to have left over the weekend and to have begun work this week.

But "there has been a delay," said a diplomat, adding that the experts "wanted assurances on the modalities (of the investigation) and they haven't received them."

Questioned about the report, UN deputy spokesman Eduardo del Buey declined to comment on the reasons for the delay. But he confirmed that the experts "are not there yet."

"We are still working on the logistical challenges," he said. "As we said last week they are gathered in The Hague and the logistics are being worked out with the Syrian authorities."

The United Nations last month reached a framework agreement with the Syrian government on the mission but has been awaiting a final green light from Damascus.

One of the sites to be investigated is Khan al-Assal, near Aleppo, where the government says rebels used chemical weapons on March 19, killing at least 26 people, including 16 Syrian soldiers.

The opposition says government forces carried out the attack.

The United Nations has not yet identified the two other sites nor said how long the team will remain in Syria.

Sellstrom is supposed to be joined by 10 experts from the Organization for the Prohibition of Chemical Weapons and the World Health Organization.

The Syrian opposition says the investigators can have access without impediment to sites under its control where chemical weapons are alleged to have been used.

Damascus had called for a UN investigation in March but insisted that it focus solely on the Khan al-Assal site. UN Secretary-General Ban Ki-moon, however, pressed for broader access and investigation of other sites as well.