In his tweet on Thursday, the president defended his warning a day earlier and appeared to try to reintroduce the element of surprise over a possible American missile strike, underscoring what critics have described as a confusing policy on United States involvement in Syria.
On Wednesday he said the missiles were "coming", but on Thursday he tweeted that he had "never said when".
On Monday, Mr. Trump said a decision on how to respond to the attacks would come in "24 to 48" hours.
In Moscow, President Vladimir Putin appealed for common sense in a world growing "more chaotic". "(That's) very meaningful for oil markets and very meaningful for geopolitical stability in the Middle East", Ayham Kamel, head of Eurasia Group's Middle East and North Africa practice, told CNBC's "Street Signs" Thursday.
US warships carrying 65-hundred sailors are on their way to the Middle East for a previously scheduled deployment.
He said there was a heightened "danger of escalation" because of the Russian military presence in Syria.
Mattis, who was spotted alongside CIA Director Mike Pompeo at the White House on Wednesday, said he stands "ready to provide military options if appropriate".
The Cabinet meeting suggests May could decide to sidestep parliamentary approval for military strikes amid reports that Assad has begun moving his arsenal away from air bases that are likely to be targeted. Among the USA military top brass "there are no madmen - these are professionals who aren't populists and know what this could lead to".
Speaking to congress at a hearing of the House Armed Services Committee on Thursday, Mattis said he believed a chemical attack had taken place and blamed Russian Federation for being complicit in Syrian government's possession of chemical weapons. France is already involved in the US -led coalition created in 2014 to fight the Islamic State group in Syria and Iraq.
The U.S., France and Britain have been consulting about launching a military strike.
"We have proof that last week, 10 days ago even, chemical weapons were used - at least chlorine - and that they were used by the regime of Bashar al-Assad", Mr. Macron said in an interview on TF1, a French television station.
President Emmanuel Macron said France, the USA and Britain will decide how to respond in the coming days.
Energy Aspects' Mallinson said Trump's reference to Iran could be viewed as further evidence the US president is not likely to renew the Iranian nuclear agreement. Syria has denied using chemical weapons. "She has said all the indications" are that Assad's forces were responsible and the use of chemical weapons "cannot go unchallenged". At the same time, she made clear that Germany wouldn't stand in the way of a response by its allies.
The eight possible targets reportedly include two Syrian airfields, a research centre and a chemical weapons facility, according to a source.
Assad meanwhile made his first comments since images of civilian victims emerged last weekend, causing indignation around the world.
"Any possible action will only cause more instability in the region and threaten global security and peace", Syrian state TV quoted Assad as saying at a meeting Thursday with a high-level Iranian delegation in Damascus.
Trump's message came roughly 24 hours after he warned on Twitter that US airstrikes "will be coming", despite a pledge from Russian Federation to shoot down American missiles aimed at its ally Syria.
Sanders says the U.S.is confident in laying blame on the Syrian government and its Russian allies for Saturday's deadly attack in Douma.
"We're maintaining that we have a number of options and all those options are still on the table".
Rescue workers and medics working in Syria said more than 40 people were killed after bombs filled with toxic chemicals were dropped by government forces in the war-torn country. Western leaders such as Macron have said the continued use of chlorine and other gas show Syria didn't totally live up to the agreement.