Families were reportedly found suffocated in their homes and shelters, with foam on their mouths.
At a photo-taking session in the Pentagon on Monday, Mattis said "the first thing" to consider in how to respond was why chemical weapons are "still being used at all".
Russian Federation proposed counter resolutions and backed a visit by United Nations weapons inspectors instead, but that was seen as a bid to avert US-led airstrikes against Syria. The larger question is whether his use of chemical weapons disqualifies him definitively for an end of the war that leaves him in power?
Last night, Trump said "nothing is off the table" in Syria and has vowed to respond "forcefully".
Asked about the possible size and make-up of potential strikes, Mr Schneider said it depends on Mr Trump's intent.
At a cabinet meeting on Monday, Trump said Putin "may" have to bear responsibility for the Syrian incident. He warned that Russian Federation or any other nation found to share responsibility in the attack will "pay a price".
"If it's Russian Federation, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out", he said.
President Donald Trump has condemned the most recent suspected chemical weapons attack in Syria. It also would establish a separate joint investigative body to determine who was responsible for any chemical weapons used in Syria.
First Warning Forecast: Falling temperatures, rain, wind and possible snow
Despite morning temps of around 10 degrees in Fargo, it wasn't cold enough to break the daily record of zero degrees set in 1881. Saturday night , the forecast calls for an overnight low of around 50 degrees but some inland areas could see the mid 40s.
Syrian President Bashar Assad on Tuesday invited global inspectors to investigate the site of the suspected attack, while Russian Federation said it would submit a UN Security Council resolution for worldwide inspectors to visit Douma.
In February, it accused Iranian forces at the T-4 base of sending a drone into Israeli territory.
"But as Barack Obama in 2013 showed us, we never really know until the moment the president gives the order" he said. The officials are not authorized to discuss military planning by name. "And there is a strategic collision that perhaps tonight may have come together because of the chemical issue".
"A small punitive strike like a year ago - basically a military slap on the wrist - would most likely hit military targets directly affiliated with the Syrian chemical weapons program: factories, warehouses or airfields".
"Syria is keen on cooperating with the OPCW to uncover the truth behind the allegations that some Western sides have been advertising to justify their aggressive intentions", state news agency SANA said, quoting an official Foreign Ministry source. The Russian military said its officers visited the hospital in Douma and talked to the staff, and said they did not confirm reports of the assault.
But neither Trump nor Obama is guilty of violating the military principle of surprise, which is critical to the success of an operation, military experts said. They said the victims had foam on their mouths.
The Syrian-American Medical Society said more than 500 people were brought to medical centres with symptoms "indicative of exposure to a chemical agent".
Iran is one of Syria's strongest backers and has sent thousands of troops and allied militiamen to support Syrian forces. The evacuations follow a pattern of departures around the capital and other major Syrian cities as the Government reasserts its control after seven years of war.
Smoke rising after Syrian government airstrikes hit in the town of Douma, in eastern Ghouta region east of Damascus, Syria, April.