WASHINGTON -- Syrian President Bashar Assad's government may be developing new, more sophisticated chemical weapons, the Trump administration says. The characteristics of recent alleged attacks suggest Syria is producing chemical weapons despite a 2013 deal to destroy its program, according to officials, who say it's "highly likely" that Syria kept a stockpile of weapons.

The officials also say Syria may be making new kinds of weapons, either to improve their military capability or to escape international accountability.

The officials also say the Islamic State of Iraq and Syria (ISIS) keeps using chemical weapons such as sulfur mustard and chlorine. The militants are using shells or improvised explosive devices to deliver the chemicals, according to the officials.

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The officials weren't authorized to discuss the assessment on the record and briefed reporters on condition of anonymity.

Fighting continues to rage on in Syria, entangled in a bloody civil war that shows no signs of being resolved in the near future. Although ISIS, which once controlled much of Syria, has been squeezed from almost all of its former territory, armed opposition groups continue to fight with each other, with Assad's forces and with extremist groups that continue to pose a threat across Syria. U.S. military forces are active both on the ground and in the skies above Syria.

More than 5.4 million people have fled the country since 2011 while millions more are displaced inside Syria, according to the U.N. Refugee Agency. Over 13 million people inside the country are in need, and nearly 3 million are in areas the U.N. describes as hard-to-reach and besieged. The conflict has put many children in serious danger.