WASHINGTON (Reuters) - The Syrian government may be developing new types of chemical weapons, and U.S. President Donald Trump is prepared to consider further military action if necessary to deter chemical attacks, senior U.S. officials said on Thursday. 
 
 FILE PHOTO: A U.N. chemical weapons expert, wearing a gas mask, holds a plastic bag containing samples from one of the sites of an alleged chemical weapons attack in the Ain Tarma neighbourhood of Damascus, Syria August 29, 2013. REUTERS/Mohamed Abdullah/File Photo 
 
 President Bashar al-Assad is believed to have secretly kept part of Syria’s chemical weapons stockpile despite a U.S.-Russian deal under which Damascus was supposed to have handed over all such weapons for destruction in 2014, the officials said. 
 
 Assad’s 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 U.S. 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. 
 
 A deadly sarin attack on a rebel-held area in April prompted 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 U.S. 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 U.S. shores,” the second official said. 
 
 “It will spread if we don’t do something,” the official warned. 
 
 The officials echoed U.S. Secretary of State Rex Tillerson’s recent accusation 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 Syrian government has said it has not carried out any of the attacks. 
 
 The U.S. 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. 
 
 Western officials have cast suspicion on the Syrian government 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. 
 
 The U.S. officials also said Islamic State militants have sporadically used chemical weapons such as sulfur mustard and chlorine, some via improvised explosive devices, a more rudimentary tactic than those of the Syrian government. 

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 The United States has accused the Syrian government of using chemical weapons against its people. 
 
 US State Department spokesman Heather Nauert said on Thursday that reports of chlorine gas being used against civilians in Eastern Ghouta were "very concerning". 
 
 "Russia is making the wrong choice by not exercising its unique influence. To allow [the] Syria regime to use chemical weapons against its own people is unconscionable. We will pursue accountability," she said, writing on Twitter. 
 
 Russia is Syria's closest ally, and has previously blocked attempts to investigate the suspected use of chemical weapons. 
 
 Nauert's comments follow a suspected chlorine gas attack in the rebel-held territory of Eastern Ghouta, on the outskirts of Damascus, on Thursday. 
 
 On Saturday, Syria's foreign ministry denied the allegations, according to Reuters news agency. 
 
 The White Helmets volunteer rescue group posted photos of the suspected attack on Twitter, under the hashtag "Douma Suffocating". 
 
 3 civilians killed and many injured, including suffocation cases among children, women and #CivilDefense volunteers! 28 air raids, 70 surface-to-surface missiles, some loaded w/chlorine gas and 200 artillery shells targeted cities in #Eastern_Ghouta today! #Doumasuffocating pic.twitter.com/yXuyWAcLMe — The White Helmets (@SyriaCivilDef) February 1, 2018 
 
 "3 civilians killed and many injured, including suffocation cases among children, women and #CivilDefense volunteers! 28 air raids, 70 surface-to-surface missiles, some loaded w/chlorine gas and 200 artillery shells targeted cities in #Eastern_Ghouta today! #Doumasuffocating", the group wrote on Twitter. 
 
 It was unclear whether White Helmets was suggesting the three people who had been killed died of suffocation. 
 
 The group claimed that volunteer Bilal Bayram was among the victims who was "suffocated by the chlorine gas attack". 
 
 Several children were also affected, the group said. 
 
 Rebel stronghold under siege 
 
 Eastern Ghouta is the last remaining rebel stronghold near the capital, Damascus, and has been under a government siege since 2013. 
 
 It is believed that around 400,000 people still live in the area, which suffers from acute food and medicine shortages. 
 
 A ceasefire deal for Eastern Ghouta was recently negotiated in Vienna by the Russians, but when implemented in late January, the truce only lasted a few hours. 
 
 "The civil war in Syria has not been at the top of the news agenda for at least weeks, but on Thursday the Trump administration made some serious concerns," said Al Jazeera's Rosalind Jordan, reporting from Washington, DC. 
 
 Off camera, reporters have been told that the US government believes the Syrian government still holds chemical weapons, Jordan said. 
 
 While the use of chemical and biological agents has been rare since World War I, in recent years the Syrian government and Islamic State of Iraq and the Levant (ISIL, also known as ISIS) have been accused of using such weapons as a tool in the conflict. 
 
 Chemical weapons have been used on several occasions during the war, which has claimed at least 400,000 lives and displaced 22 million people since fighting began in 2011. 
 
 Last April, autopsy results revealed that chemical weapons were used in an attack which killed more than 80 people in Syria's Idlib province. Syrian President Bashar al-Assad said that suspected attack was a "fabrication" to justify a US military attack.
– The Syrian regime not only kept some of its chemical weapons after a 2013 disarmament deal, it has apparently "evolved" new types of weapons and continues to use them on rebel-held civilian areas, Trump administration officials warn. The officials, speaking under condition of anonymity, tell Reuters that the administration hopes to deal with the Assad regime through sanctions and diplomatic pressure, but another military strike is also an option—especially since there's the risk of the Syrian weapons spreading to US shores. "We reserve the right to use military force to prevent or deter the use of chemical weapons," one official says. The US hit a Syrian air base with dozens of cruise missiles last April after a chemical attack killed dozens of civilians. "We're certainly seeing the evolution of allegations into new kinds of weapons that suggests an ongoing production capability" of sarin and chlorine gas, one official tells the Washington Post. "They clearly think they can get away with this if they keep it under a certain level." Al Jazeera reports that a State Department spokeswoman issued a warning after a suspected chemical attack on civilians Thursday in Eastern Ghouta, a besieged rebel-held area near Damascus. The reported use of chlorine gas is "very concerning," she tweeted. Russia "is making the wrong choice by not exercising its unique influence," she added. "To allow #Syria regime to use chemical weapons against its own people is unconscionable. We will pursue accountability." (The "capital of the revolution" is now largely abandoned.)