A Downing Street spokesman said: "The Prime Minister spoke to the French President Emmanuel Macron this evening about the horrific attack in Douma, Syria on Saturday".
Trump said the U.S.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.
Trump said that the "massacre" last weekend in Syria "was a significant escalation in a pattern of chemical weapons use by that very bad regime".
"Again, we are being threatened".
On the suspected poison gas attack on Douma, Ms Haley said: "We know who did this".More news: The Heat: Summit of the Americas
"The threats of some Western countries to attack Syria is based on lies that these countries fabricated along with terrorist organisations", Assad's office said.
Officials also said the strike was larger than a similar attack on the Assad regime launched a year ago. "Russia was supposed to guarantee Assad would not use chemical weapons, and Russian Federation did the opposite". "These are not actions of a man, they are crimes of a monster instead".
Several explosions were heard in the capital of Damascus. Barzah is the location of a major Syrian scientific research center. 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 attack brought to mind the 2017 Shayrat missile strike that took place in April 2017, and involved the launch of 59 Tomahawk cruise missiles by the United States from the Mediterranean Sea into Syria, aimed at the Shayrat Airbase controlled by the Syrian government.
Russian Federation has called for a United Nations Security Council meeting on Syria on Friday.More news: Prosecutors: Probe of Trump lawyer Michael Cohen centers on Cohen's business dealings
Mattis said the initial strike was completed.
Trump also talked up the United States efforts to eradicate ISIS in the Middle East, suggesting countries should be more appreciative of America.
Appearing before the House Armed Services Committee, US Defense Secretary Jim Mattis told lawmakers that the use of chemical weapons was "inexcusable".
Trump also addressed the governments of Iran and Russian Federation: "I ask what kind of a nation wants to be associated with the mass murder of innocent men, women and children".
The British Prime Minister, Theresa May, was convening a meeting of her Cabinet on Thursday afternoon, at which she is expected to make the case for supporting the U.S. in any military action against Syrian President Bashar al-Assad.
Mr Turnbull criticised Russian Federation for its veto of a UN Security Council resolution this week that sought to respond to reports of chemical weapon attacks.More news: Bases evacuated on Russian warning: Syria