US President Donald Trump has said that he will decide on potential actions against Syria within 24-48 hours in response to reports of an alleged chemical attack in Douma, East Ghouta.

Trump promised a swift response to the "heinous attack," saying that "this is about humanity and it can’t be allowed to happen.”

"He may," President Trump says when asked if Russian President Putin bears responsibility for alleged chemical attack in Syria. "And if he does, it’s going to be very tough.” pic.twitter.com/2tAUDwNFst — MSNBC (@MSNBC) April 9, 2018

His statements echo earlier remarks by Defense Secretary James Mattis. When asked by a reporter if the US was considering military strikes against Syria, Mattis replied that he "would not rule out anything right now."

On Sunday, Trump wrote on Twitter that there would be a "big price to pay" for the alleged attack.

Moscow says the accusations against Damascus serve as a baseless provocation. It has warned that any military action taken to 'respond' to the alleged incident would have "dire consequences." Trump used similar language after an alleged chemical attack was carried out in Idlib province last April. He later ordered a Tomahawk missile strike against a Syrian airbase in Homs.

READ MORE: Enter Bolton: Trump’s new security adviser comes with chemical casus belli on hand

The US State Department said in a statement released on Monday that victims of the alleged attack in Douma showed symptoms of exposure to a nerve agent, and called on Syria and Russia to open the area to international monitors.

Saturday’s alleged chemical attack in Douma, Eastern Ghouta, comes as pro-government forces conclude an operation that has liberated nearly the entire region from Islamic militants. The alleged chemical incident was reported by pro-jihadist groups and it could not be confirmed by Red Crescent medics on the ground.Following the reports, two Israeli warplanes targeted a Syrian airbase in Homs on Sunday night, according to the Russian and Lebanese militaries.

Trump's bluster last week about withdrawing from Syria meant nothing. US always wants to be seen as the reluctant superpower, dragged into conflicts against its will. "All we want to do is go home & rest on our laurels but the bad guys keep forcing us to intervene everywhere." — George Szamuely (@GeorgeSzamuely) April 9, 2018

Monday also marks John Bolton’s first day as Trump’s new national security advisor. Bolton, a former Bush-era ambassador to the UN, has been a vocal proponent of more US military involvement in the Middle East.