The Organization for the Prohibition of Chemical Weapons, based in the Netherlands, announced it would send a fact-finding team to the site of the attack outside Damascus, and it was due to arrive Saturday.
Some MPs have backed Britain acting against Syria, warning that the use of chemical weapons was in breach of worldwide law and could not be allowed to go unpunished.
The United States accused the Syrian government on Friday of using banned chemical arms at least 50 times since Syria's civil war began seven years ago - far higher than the number of attacks that had been publicly confirmed.
"With every victory achieved on the field, the voices of some Western states are raised and actions are intensified in an attempt by them to change the course of events. these voices and any possible action will contribute nothing but an increase in instability in the region, threatening worldwide peace and security", Mr al Assad said according to Syrian state television.
"In the past, Trump has condemned others for forecasting military plans, repeatedly blistering President Barack Obama during the 2016 campaign".
Syria and Russia have denied using poisonous gas in Douma on April 7, with Russian Foreign Minister Sergei Lavrov saying that Moscow had evidence that the attack in Douma was staged.
USA officials have refused to rule out direct military engagement with Russian Federation, with the White House saying "all options are on the table".
The possibility of a military strike on Syria was discussed by the United States and its partners, particularly France and the United Kingdom, in the wake of reports on an alleged chemical weapons attack in the Syrian city of Duma.
President Donald Trump on Thursday softened his rhetoric about potential airstrikes on Syria, a day after warning Russian Federation that missiles "will be coming, nice and new and 'smart'". Trump tweeted Wednesday that missiles "will be coming" and on Thursday tweeted that an attack "could be very soon or not so soon at all!"
Trump has since tempered those remarks and the White House said no final decisions on possible actions had been taken.
His latest tweet, on Thursday evening, said: "Never said when an attack on Syria would take place".
Macron said Thursday that France has proof the Syrian government launched chlorine gas attacks and said France would not tolerate "regimes that think everything is permitted". 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. In Syria, this instinct points him away from a military-centric answer to Assad, writes Bonnie Kristian.
"You are a minority Government, and you need to seek the consent of Parliament before you commit the United Kingdom to any action".
"Japan can not make a judgment on whether Assad's government forces really used chemical weapons", a senior Foreign Ministry official said of the latest attack. France is already involved in the USA -led coalition created in 2014 to fight the Islamic State group in Syria and Iraq. Multiple Islamic State retaliatory attacks have targeted French soil, including just last month.