The United States accused on Thursday the Syrian regime of continuing on developing chemical weapons in what is a violation of an agreement to abolish its program in 2013. Senior US officials said that the regime of Bashar Assad is believed to have secretly kept part of Syria’s chemical weapons stockpile despite a US-Russian deal under which Damascus was supposed to have handed over all such weapons for destruction in 2014, the officials said. Regime forces have instead “evolved” their chemical weapons and made continued occasional use of them in smaller amounts since a deadly attack last April that drew a US missile strike on a Syrian air base, the officials told reporters in a briefing. Characteristics of some of those recent attacks suggest that Syria may be developing new weapons and methods for delivering poison chemicals, possibly to make it harder to trace their origin, the officials said, speaking on condition of anonymity, but they declined to provide specifics. Barrel bombs used earlier in the war to disperse chemicals indiscriminately, for example, have been replaced by ground-launched munitions, officials said. More recent attacks have involved both chlorine, which has non-chemical uses and is easier to acquire, and the more sophisticated chemical sarin, the officials said. A deadly sarin attack on a rebel-held area in April prompted US President Donald Trump to order a missile strike last year on the Shayrat air base, from which the Syrian operation is said to have been launched. “We reserve the right to use military force to prevent or deter the use of chemical weapons,” one official said, while declining to specify how serious a chemical attack would have to be to draw a fresh US military response. A second official said, however, that the Trump administration hopes that stepped-up international sanctions and diplomatic pressure will help rein in Assad’s chemical weapons program. If the international community does not act quickly to tighten the screws on Assad, Syria’s chemical weapons could spread beyond its borders and possibly even “to US shores,” the second official said. “It will spread if we don’t do something,” the official warned. Western officials have cast suspicion on the Syrian regime for a chlorine gas attack on a rebel-held enclave east of Damascus last week that sickened at least 13 people. State Department spokeswoman Heather Nauert said on Thursday the United States was “extremely concerned” about reports that Syrian forces had carried out another chlorine gas attack this week in the Eastern Ghouta area. US Secretary of State Rex Tillerson recently said that Russia, Assad’s ally in Syria’s multi-sided civil war, bears some responsibility for failing to enforce the chemical weapons ban. Russia has denied any complicity, and the regime has said it has not carried out any of the attacks. The US officials suggested that if left unchecked there would be more smaller chemical attacks as an “instrument of terror” to compensate for Assad’s lack of adequate manpower to retake some opposition-held areas. “They think they can get away with it if they keep it under a certain level,” an official said. Reports of chemical attacks have continued to stream in from Syria, including as recently as Thursday, when rescue workers in the rebel-held Damascus suburb of Douma reported what they described as a suspected chlorine gas attack that injured a number of civilians. The opposition-run Ghouta Media Center reported in a posting on its Facebook page that three people were killed and dozens suffered shortness of breath as a result of surface-to-surface missiles, some of them carrying chlorine gas.