Senior officials said during a White House conference call Saturday that social media posts and eyewitness accounts of a suspected chemical attack justify President Trump's decision to attack Syria's government.

Officials denounced people who question whether Syrian President Bashar Assad killed about 40 people with an April 7 gas attack on Douma, the final rebel-held Damascus suburb.

Trump ordered 105 missiles fired overnight at three Syrian government facilities, in cooperation with the British and French militaries.

“There is a narrative, a Syrian regime and Russian-fueled counter-narrative out there where people tend to say for example, well, the supposition that the Assad regime is militarily winning so it wouldn’t have any motivation to use chemical weapons," one official said on the call.

"We see people who should know better actually making that argument," the official said on the call.

The official said that contrary to widespread belief, Assad is not on the cusp of defeating rebels after more than seven years of civil war.

"In reality, first of all, the Assad regime is not winning strategically," the official argued. "If you look at the situation in Syria, they control about half of Syria’s populated territory, about half of its population and less than half of its prewar GDP. They may be winning militarily in some places operationally, but they’re winning in those places operationally in part because they use chemical weapons."

The official added: "It’s not that because they’re winning they would not have a motivation to use chemical weapons. It’s that they're using their chemical weapons as part of their way to win."

The official said Assad's government used chemical weapons against Douma "in order to preserve its military manpower."

Opponents of a military strike argue it's improbable that Assad would use chemical weapons, knowing it would invite an international response, when he's on course to defeat the few remaining rebel-held pockets in Syria.

“Anybody who has tried to explain to me why Assad would do this sounds like a conspiracy theorist,” Rep. Thomas Massie, R-Ky., told the Washington Examiner on Thursday. “It requires really contorted thinking to think Assad did this. You have to come up with very complicated reasons.”

Meanwhile, Russia, which has provided indispensable military support to Assad since 2015, has accused the United Kingdom of staging the incident to provide a pretext for the assault.

A second official pointed to photos and social media posts as evidence of Assad's guilt.

"We have incontrovertible evidence from the photos," the second official said, referring to videos and photos from social media, non-governmental organizations, and other open source outlets.

The same official said “multiple eyewitnesses” reported government helicopters hovering before the alleged attack and that “numerous eyewitnesses also corroborate the story that barrel bombs were dropped.” After, the official said, “Doctors described a strong smell of chlorine and described symptoms consistent with sarin.”

“It’s unlikely any opposition group would actually have the capability to fabricate the large amount of information that we have and that is available publicly that reports the regime chemical weapons use," the official said. "This kind of fabrication would require a highly organized and compartmentalized campaign to deceive multiple media outlets while evading our detection. It just isn't feasible. The Syrian regime and the Russians have also claimed that terrorist groups conducted the attacks or the attacks were staged. As I noted, this just isn't consistent with the available body of credible information."

She added: "No non-state actors conducted air operations throughout the conflict. This further implicates the regime."

Earlier on Saturday, Pentagon officials at a press conference side-stepped questions about evidence that Syria’s government was responsible.

Defense Secretary James Mattis said Thursday morning the U.S. had no evidence of a chemical weapons attack. At a Friday night press conference, Mattis said he became convinced later on Thursday that the U.S. had evidence.

Trump ordered the bombing of Syria once before for a chemical weapons attack, in April 2017 launching 59 Tomahawk missiles against an airbase in response to the alleged use of sarin gas against a town in northern Idlib province. Mattis said in February that the U.S. still did not have evidence of Assad's guilt in that incident.

Officials on the White House call did not address the legal basis for Trump attacking Syria.

The White House officials stood by the Pentagon's account that Russians were not alerted to the specific bombing targets, and echoed that the strike would be a one-time strike to deter future chemical weapon use.