The Syrian Foreign Ministry has responded to the United States' claims of chemical weapons use by the government forces in the clashes with militants in Eatern Ghouta.
The U.S. has no evidence to confirm reports from aid groups and others that the Syrian government has used the deadly chemical sarin on its citizens, Defense Secretary Jim Mattis said Friday.
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.
The officials also say Syria may be making new kinds of weapons, either to improve their military capability or to escape global accountability.
Washington said this week that it could consider military action against the Syrian government if it continued to use chemical weapons.
The secretary of state said must stop vetoing U.N. Security Council resolutions on holding those who use such weapons accountable.
Secretary of State Rex Tillerson said at the time the USA had "very high confidence" that the chemical agent used in that deadly attack was sarin nerve gas.
"We do not have evidence", Mattis told reporters, as quoted by Politico on Friday, adding that it appears chlorine was used in the attacks, but "we are even more concerned about the possibility of sarin use".
"Claims that the Syrian state used chlorine gas one moment and sarin gas the next prove that these are nothing more than lies", the statement said.
President Donald Trump said the attack was meant to deter further Syrian use of illegal weapons.
The April 2017 attack on Khan Sheikhun left scores dead and prompted the U.S. to fire 59 Tomahawk cruise missiles at a Syrian air field.
A deadly sarin attack on another rebel-held area in April 2017 prompted President Donald Trump to order a U.S. missile strike on the Shayrat airbase, from which the Syrian operation is said to have been launched.