(CNN) -- A large portion of Syria's remaining declared chemical weapons stockpiles has been neutralized, a chemical weapons watchdog group said Wednesday.
Workers aboard a U.S. government container ship finished neutralizing 581 metric tons of methylphosphonyl difluoride, a chemical used to make nerve agents sarin and soman, the Organisation for the Prohibition of Chemical Weapons said.
The ship, the Cape Ray, still needs to neutralize 19.8 metric tons of sulfur mustard, a blistering agent. Once that is done, the last batch of declared Syrian chemical weapons to have left the country will have been rendered useless.
Other batches of Syria's chemicals are being destroyed elsewhere, including at facilities in Britain and Finland. Last week, the OPCW said that more than 74% of Syria's declared chemical stockpile had been destroyed.
Syria agreed to give up is chemical weapons last year as part of an international deal meant to stave off Western military strikes, after Syria's government was accused of using poison gas against civilians during its years-long civil war.
In June, when a Danish ship collected the final declared batch of methylphosphonyl difluoride and sulfur mustard from the Syrian port of Latakia, the OPCW hailed what it called a major milestone.
"Never before has an entire arsenal of a category of weapons of mass destruction been removed from a country experiencing a state of internal armed conflict," OPCW chief Ahmet Uzumcu said at the time.
But inspectors can't say for sure that some undeclared weapons aren't still lurking somewhere in the country.
The Danish ship transferred the chemicals to the Cape Ray at an Italian port in July. The Cape Ray, equipped with chemical neutralization systems, began processing the chemicals after sailing to international waters in the Mediterranean Sea.
The neutralized chemicals will be destroyed at facilities in Finland and Germany, the OPCW said.
While the OPCW says Syria has given up all the chemical weapons that it declared, the country's civil war continues. The United Nations says more than 150,000 people have been killed in the past three years.
The war began after popular protests against Syrian President Bashar al-Assad in 2011. Syrian government forces are now opposed by various groups, including the extremist Sunni Muslim group Islamic State in Iraq and Syria, or ISIS. That militant group, which calls itself the Islamic State, now controls parts of northeastern Syria and northern and western Iraq.
CNN's Carol Jordan and Ben Brumfield contributed to this report.