The first batch of the most dangerous materials in Syria’s banned chemical weapons stockpile was exported from the country on Tuesday, loaded onto a Danish commercial vessel in the Syrian port of Latakia in an operation overseen jointly by the United Nations and the group responsible for ensuring the arsenal’s destruction.

In a statement, the United Nations and the Organization for the Prohibition of Chemical Weapons, The Hague-based group that monitors the treaty Syria agreed to join in September, said the Danish vessel had departed Latakia and would remain at sea until the second cargo of chemicals reaches Latakia, when it will return to load them. The vessel was escorted by Danish and Norwegian naval vessels, the statement said, and China and Russia were providing further maritime security for the operation.

“This movement initiates the process of transfer of chemical materials from the Syrian Arab Republic to locations outside its territory for destruction,” said the statement by Sigrid Kaag, the United Nations official responsible for coordinating the effort.

The export and destruction of the most dangerous substances in the Syrian arsenal, which the statement called “priority chemical materials,” has long been considered the trickiest and most hazardous part of the operation, which Syria agreed to carry out as part of its pledge more than three months ago to renounce chemical weapons and join the treaty that bans them.