Trump's tweet follows a threat he issued on Wednesday for Russia to "get ready" because the USA would soon strike Syria, regardless of Russian claims that such a strike would be met with a Russian counter-attack.
He said the missiles would be "new and 'smart!'"
But she had also made clear the president was holding both Syria and Russian Federation responsible for the attack. Russia signaled its alliance with Syria will continue, with Russian ambassador to Lebanon Alexander Zasypkin warning the president Tuesday that any missiles fired at Syria will be shot down.
Israel has said it won't let Iran entrench militarily in Syria, and Prime Minister Benjamin Netanyahu took the occasion of Israel's Holocaust Remembrance Day to warn Tehran not to test his country. Satellite images released by the Israeli company ImageSat International showed that Russian ships had deployed from Russia's naval base at the Syrian city of Tartus.
"The objective of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons", Trump said. At least 40 people are said to have died. "Air strikes have not prevented these attacks and will not provide the long-term solutions needed to end the war", he said. "There's somebody who wants to stir up the trouble, and the people who want us to stay there".
The U.N. agency demanded that it be given the "immediate, unhindered" ability to meet with people injured in the attack.
The demonstrations grew for months until President Bashar al-Assad sent the military in to crush them. But Syria and Russian Federation have denied that charge. Trump canceled a foreign trip in order to manage a crisis that is testing his vow to stand up to Assad.
On the suspected poison gas attack on Douma, Ms Haley said: "We know who did this".
The remaining rebels in Syria's battered Eastern Ghouta have surrendered their heavy weapons and their leader has left the enclave for the north, a war monitor said on Thursday.
"We're still assessing the intelligence, ourselves and our allies", Mattis said.
Efforts to find a diplomatic solution at the UN Security Council Tuesday failed, with Washington and Moscow opposing each other's rival motions to set up an worldwide investigation into chemical weapons use. The U.S. and its allies said Assad's government had used sarin gas.
The United States, Britain and France have argued the incident bears all the hallmarks of a strike ordered by the regime, which has been blamed for previous attacks by the Organisation for the Prohibition of Chemical Weapons (OPCW).