The transfer of Syrian chemical weapons from a Danish container ship to a US vessel was completed on Wednesday in an Italian port, the Pentagon said in a statement.

The disposal process marks the culmination of a program to rid Syria of its chemical weapons stockpile after the outcry that followed chemical attacks by the Bashar al-Assad regime in the suburbs of Damascus on August 23 last year,that may have killed as many as 1,400 people.

"The transfer of Syrian chemicals from the Danish container ship Ark Futura to the Motor Vessel Cape Ray is complete," said Pentagon press secretary Rear Admiral John Kirby.

"Cape Ray departed the Italian port of Gioia Tauro this afternoon for international waters in the Mediterranean Sea, where neutralization operations will soon begin.

"The neutralization process should take several weeks to complete."

Hundreds of tonnes of mustard gas and ingredients to make Sarin nerve gas were transferred from the Danish vessel in the southern Italian port amid tight security.

"Secretary Hagel is grateful to Danish and Italian authorities for their support in this process and is enormously proud of everyone who helped make possible this safe and incident-free transfer," Kirby said.

"He extends a special thanks to the men and women of the Cape Ray, Naval Forces Europe, and US European Command teams for their impeccable planning and execution."