WASHINGTON (Reuters) - The United States is concerned about the potential use of sarin gas in Syria, U.S. Defense Secretary Jim Mattis said on Friday, a day after Washington warned it was prepared to consider military action if necessary to deter chemical weapons attacks by the Syrian government.

FILE PHOTO: U.S. Defense Secretary James Mattis waits to welcome British State Secretary for Defense Gavin Williamson to the Pentagon in Arlington, Virginia, U.S., February 1, 2018. REUTERS/Yuri Gripas

Mattis, speaking with reporters, said the Syrian government had repeatedly used chlorine as a weapon. He stressed that the United States did not have evidence of sarin gas use.

“We are even more concerned about the possibility of sarin use, (but) I don’t have the evidence,” Mattis said. “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 did not provide further details on which reports on the use of sarin he was referring to.

However, the Syrian Negotiations Commission (SNC), an opposition group, said chemical weapons had been used by the government of Bashar al-Assad in Ghouta.

Rescue workers and medical groups working in the rebel-held enclave of Eastern Ghouta, near Damascus, have accused government forces of using chlorine gas three times over the last month, including early on Thursday.

“Chemical weapons are being used in Ghouta and we have proof,” SNC spokesman Yahya al-Aridi told Reuters.

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.

“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.

France said on Friday it was “deeply concerned” that Syria’s government was flouting its pledges to stop using chemical weapons and Paris was working with its partners to shed light on recent suspected toxic gas attacks.

In 2013, Syria said it would give up all of its chemical weapons. In the past two years, a joint United Nations and Organization for the Prohibition of Chemical Weapons (OPCW) inquiry found the Syrian government used the nerve agent sarin and several times used chlorine as a weapon.

U.S. officials have said the Syrian government may be also developing new types of chemical weapons.

The Syrian army and government have consistently denied using chlorine or other chemical weapons during the war and has said it cooperates with international investigations.