The United States is investigating whether the Syrian government recently used sarin gas in attacks on the opposition, US Secretary of Defense Jim Mattis said Friday.

"We have other reports from the battlefield from people who claim it's been used," Mattis said, adding the US currently had no proof to back the claims.

"We're looking for evidence of it, since clearly we are dealing with the Assad regime that has used denial and deceit to hide their outlaw actions," he said.

Read more: US blames Russia for Syria chemical weapons attacks

The US launched airstrikes against a Syrian government military base last April after more than 80 people were killed in a sarin gas attack on a town held by the Syrian opposition.

"You all have seen how we reacted to that, so the Syrian government would be ill-advised to go back to violating the chemical convention," Mattis said.

Read more: UN — Syrian regime used chemical weapons more than two dozen times

Chlorine gas used

Mattis told reporters the Syrian government had committed multiple attacks using chlorine gas. Unlike sarin, chlorine gas is relatively easy to produce and is not banned by the Chemical Weapons Convention.

On Thursday, the US State Department said it was "extremely concerned" about reports of an imminent chlorine gas attack against an opposition-held suburb outside of Damascus.

The AFP news agency also reported senior officials had said the US was not ruling out military force in response to future chemical weapons attacks by the Syrian government.

Read more: Russia vetoes last-ditch bid to save probe into gas attacks in Syria

Syria had agreed in 2013 to give up its chemical weapons to the Organization for the Prohibition of Chemical Weapons (OPCW).

But the UN and the OPCW have said the Syrian government has used sarin and chlorine gas weapons in the past two years, an accusation Syria has repeatedly denied.

Read more: UN — Syrian regime used sarin gas in April attack

amp/cmk (dpa, Reuters, AP)