Also, French President Emmanuel Macron said he had ordered a military intervention in Syria alongside the United States and Britain in an attack on the chemical weapons arsenal of the country's regime.
"This massacre was a significant escalation in a pattern of chemical weapons use by that very awful regime", Trump said.
Both Syria and Russian Federation have said reports of the attack were fabricated by rebels and rescue workers in the town and have accused the US of seeking to use it as a pretext to attack the government.
The combined US, British and French assault appeared to be more intense than a similar strike Trump ordered nearly exactly a year ago in retaliation for a chemical weapons attack attributed to Assad. "We thank them both".
Russian Federation has hit back at the United States following a massive air strike on targets in Syria, warning there will be consequences.
British Prime Minister Theresa May won backing from her senior ministers on Thursday to take unspecified action with the United States and France to deter further use of chemical weapons by Syria. Syrian state TV says the attack has begun on the capital, though it wasn't immediately clear what was targeted.
Defense Secretary James Mattis and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford met with the president at the White House on Wednesday to discuss the situation in Syria.
"We can not tolerate the recurring use of chemical weapons, which is an immediate danger for the Syrian people and our collective security", a statement from the Elysee presidential office said. The Syrian president on Wednesday made a public appearance with a group from an worldwide Muslim body in Damascus. The Syrian government has been known to use chlorine bombs, along with sarin, a nerve agent developed by Nazi Germany that was used in a 2013 attack that killed almost 1,500 people outside of Damascus.
The Prime Minister has won the backing of her Cabinet for action to prevent further chemical attacks by Syria's President Bashar al Assad.
Russian Federation is a major backer of the government of Syrian President Bashar al-Assad in the country's civil war, and strongly opposes the USA claim that Syrian government forces attacked a rebel-held town near Damascus on Saturday with chemical weapons. But during the weekend, images of dead and sick women and children again circulated following another alleged chemical attack.
Denmark supports attack on Syria … but not militarily
More broadly, he doubted the wisdom of bombing. "This is exactly the risk we face today - that things spiral out of control". Britain has launched air strikes against Islamic State militants in Syria, but not against the country's government.
This is not about intervening in a civil war.
Top-level Israeli security officials met Wednesday to prepare for scenarios in which Israel is attacked by Iran or Syria, following a tweet by U.S. President Donald Trump declaring he would bomb Syria.
"We did everything we could in our intelligence assessment and our planning to minimise to the maximum degree ... any chance of civilian casualties". Military, economic and diplomatic. "I also have a message tonight for the two governments most responsible for supporting, equipping, and financing the criminal Assad regime".
Speaking from the White House, Trump says, "To Iran and to Russian Federation, I ask: What kind of a nation wants to be associated with the mass murder of innocent men, women and children?"
"The nations of the world can be judged by the friends that they keep", he continued.
He criticized Russia's support of the Syrian regime saying "Russia must decide if it will continue down this dark path".
"A pre-designed scenario is being implemented".
The Organisation for the Prohibition of Chemical Weapons tweeted Thursday that a team of its investigators will start work Saturday in Douma.
In congressional testimony this week Mattis addressed some of the concerns administration officials weighed as they deliberated on a possible response.
EARLIER: US President Donald Trump has confirmed "precision strikes" against Syria are underway.