Italy's Ministry of Interior noted that the transfer of the chemicals from a Danish ship to the U.S. government container ship, the Cape Ray, was underway at the Italian port of Gioia Tauro.
The Pentagon said on Tuesday that the process could take two to three days.
Once the transfer is completed, half of the 1,300 metric tons of chemicals aboard the Cape Ray will be neutralized in international waters in a process expected to take 60 days.
Neutralization occurs when the chemicals are mixed with water and sodium hypochlorite bleach to produce a very low-level waste.
The second half of the chemicals (mustard gas and methylphosphonyl difluoride) will be destroyed at plants in the U.S. and Europe.
In early December 2013, the U.S. Department of Defense announced it had offered a technical solution to the Organization for the Prohibition of Chemical Weapons for removing and destroying Syria's chemical weapons.
OPCW inspectors are expected to be aboard the Cape Ray to verify the chemical weapons have been neutralized.