The president made his latest statement in a tweet Thursday morning. "Could be very soon or not so soon at all!" Russia, a key ally of Assad, says government forces have taken full control of the town of Douma, the last rebel holdout in the region and the scene of Saturday's alleged attack.
After inviting investigators to the country this week, state-run SANA quoted a foreign ministry official saying: "Syria is keen on cooperating with the OPCW to uncover the truth behind the allegations that some western sides have been advertising to justify their aggressive intentions".
In April previous year Trump ordered Tomahawk strikes on the Shayrat Airbase in response to a similar chemical weapons attack on rebel-held Khan Sheikhun.
In his tweet on Wednesday, Trump referred to Assad as a "gas killing animal".
"We're still assessing the intelligence, ourselves and our allies", Mattis said.
Trump suggested Monday he had little doubt that Syria was to blame, but neither he nor other administration officials have produced hard evidence. Unless some way can be found to harm his military significantly, without danger of wider USA involvement, that may be the only option.
In the past, Trump has condemned others for forecasting military plans, repeatedly blistering President Barack Obama during the 2016 campaign. We are totally predictable.
Moscow and the Syrian regime deny that the gas attack took place or that either regime was involved.
Rep. Adam Smith, the ranking Democrat on the House Armed Services Committee, said he sees no legal justification for a USA strike in Syria, absent explicit authorization by Congress.
Sanders adds that US officials are "continuing to assess intelligence" and are "engaged in conversations with our partners and allies".
The U.S. and its allies threatened military action after an alleged gas attack by government forces over the weekend that Syrian opposition activists and medics say killed more than 40 people. There is no reason for this, he wrote, adding that "Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together".
"We call upon. members of the global community to seriously consider the possible consequences of such accusations, threats and especially action (against Syria)", Russian foreign ministry spokeswoman Maria Zakharova said.
Trump's administration has sought to show toughness on Russian Federation, with a series of economic and diplomatic actions, including new sanctions last week against government officials and oligarchs.
Turkish President Recep Tayyip Erdogan says he will discuss ways of ending "the chemical massacre" in Syria during a telephone call with Russia's President Vladimir Putin.
The U.S., France and Britain have been consulting about launching a military strike. "It was understood the United Kingdom was planning a genuinely joint effort with the USA and France, rather than playing a supporting role", said the BBC's Laura Kuenssberg.
"She has said all the indications" are that President Bashar Assad's forces were responsible, and the use of chemical weapons "cannot go unchallenged".
Conservative former London mayoral candidate Zac Goldsmith, tweeted: "We need a clear response to the Syrian chemical outrage". On Tuesday, Macron said any French action would target Syria's chemical weapons abilities.
Half a world away in New York, Russia's United Nations ambassador warned the priority in Syria was to avert US-led strikes that could lead to a risky confrontation between the world's two preeminent nuclear powers.
"On 7 April, probably the last attempt to stage a chemical weapons attack in Eastern Ghouta was made", Lieutenant General Viktor Poznikhir was quoted as saying in the state-run TASS news agency this week.