WASHINGTON (Reuters) - U.S. Defence Secretary Jim Mattis said on Friday that while he does not have evidence of the nerve agent sarin being used by the Syrian government, the United States was looking into reports about its use and was concerned.
Mattis, speaking with reporters, said the Syrian government had repeatedly used chlorine as a weapon.
"We are even more concerned about the possibility of sarin use... I don't have the evidence, what I am saying is, that other groups on the ground, NGOs, fighters on the ground, have said that sarin has been used, so we are looking for evidence," Mattis said.
A deadly sarin attack on a rebel-held area in April prompted 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 are on the record and you all have seen how we reacted to that, so they would be ill-advised to go back to violating the chemical (weapons) convention," Mattis said.
The Syrian government has said it has not carried out any of the attacks.
U.S. officials have said the Syrian government may be developing new types of chemical weapons, and President Donald Trump is prepared to consider further military action if necessary to chemical attacks.
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.
(Reporting by Idrees Ali; Editing by Chizu Nomiyama and Andrew Hay)