Updated

Syrian President Bashar al-Assad may be developing new chemical weapons in addition to keeping some of the country’s stockpile that was supposed to be handed over and destroyed under a 2014 deal brokered by the United States and Russia, senior U.S. officials told reporters on Thursday.

Officials said that Assad may have secretly kept part of Syria’s stockpile, and recent attacks by Damascus have led them to believe that Syria may be developing new weapons entirely, Reuters reported.

Declining to give further information, officials said Syria may be developing a new delivery method that would make it harder to trace the origin of the attacks.

President Donald Trump ordered a missile strike against a Syrian airbase last April after Assad used chemical weapons to attack dozens of civilians, including women and children.

However, a senior official said the Trump administration is hopeful this time that more sanctions and diplomatic pressure will help eliminate Assad’s chemical weapon program, Reuters reported.

“It will spread if we don’t do something,” the official warned.

The State Department said Thursday that the U.S. was “extremely concerned” over reports of another chemical attack in the opposition-controlled region of Damascus.

“We will continue to seek accountability through all available diplomatic mechanisms, including the United Nations Security Council and the Organization for the Prohibition of Chemical Weapons, for the confirmed use of chemical weapons by any party,” the State Department said in a statement.

Russia recently has been singled out for allowing Syria room to maneuver.

Secretary of State Rex Tillerson has said that Russia bears responsibility for the attacks for failing to enforce the weapons ban.