He added that a combined military operation by the USA, the United Kingdom and France is already underway in Syria.
And last year's strikes against Syria don't seem to have deterred the Syrian government from using chemical weapons again.
Trump said Washington is prepared to "sustain" pressure on Assad until he ends what the president called a criminal pattern of killing his own people with internationally banned chemical weapons.
The Syrian government, which denies the chemical attack, said the strikes were brutal, barbaric aggression by western powers and vowed to continue to restore security and stability in all corners of the country.
President Trump appears to have been vindicated in his claim that the federal intelligence agencies and the FBI wiretapped him and his campaign team in an attempt to influence the 2016 presidential election and deprive him of credibility.
The U.S. left what was then a 12-nation pact in January 2017 during Trump's first full weekday in the Oval Office.
"The use of chemical weapons by anyone, anywhere, under any circumstances is illegal and utterly reprehensible".
We can not tolerate the normalisation of the use of chemical weapons, he said in a statement.
Loud explosions ripped through Syria's capital early Saturday and the sky turned orange as Syrian air defense units fired surface-to-air missiles in response to joint airstrikes by the United States, France and Britain.
"To Iran and Russian Federation I ask, what kind of nation wants to be associated with the mass murder of innocent men, women and children?" he said.
The gas allegedly used in the Duma attack is chlorine. Officials said the plan to hit three targets in Syria was aimed at avoiding "collateral damage", including Russian assets.
Russian officials and Trump have exchanged heated rhetoric since Trump began talk of a USA response to the alleged chemical attack throughout the past week.
The approach, Biddle said, reflects Trump's roots as a real estate developer who made risky bets and went bankrupt several times on the way to making billions of dollars.
He said that Australian agricultural exporters in particular are at an advantage compared to their American colleagues, "and I think that's one of the reasons why the United States is having a second look at possibly rejoining the TPP".
Taro Aso, Japan's finance minister and deputy prime minister said that "if the news is true, we would welcome it". Now, they said, his orders were being carried out.
"On Saturday, April 7th, at 7:45 PM local time, amidst continuous bombardment of residential neighbourhoods in the city of Douma, more than 500 cases -the majority of whom are women and children- were brought to local medical centers with symptoms indicative of exposure to a chemical agent".
Senator Orrin Hatch, R-Utah, said in a statement that the president's decision "reflects his seriousness in addressing the scale and depravity of Assad's actions". A sarin gas attack a year ago killed more than 80 people in the town of Khan Shaykhun, and two days later Trump authorized launch of dozens of cruise missiles on a Syrian air base.
While it will take some time for an ultimate decision to come down from the White House on TPP, Priest said the renewed interest is a positive sign for the shoe industry. Trade advisers say the department could use a program known as the Commodity Credit Corp.to purchase potentially billions of dollars of crops from American farmers harmed by tariffs.