Washington: US President Donald Trump said an apparent poison gas attack in Syria that killed more than 70 people has changed his thinking on the six-year crisis, as the US signalled a more aggressive response to a conflict that Western leaders and the UN have struggled to resolve.

Asked on Wednesday if the gas attack crossed a "red line," Mr Trump said it "crossed a lot of lines for me. That crosses many, many lines, beyond red lines." Standing alongside King Abdullah II of Jordan at the White House, he called the incident "an affront to humanity. These heinous actions by the Assad regime cannot be tolerated."

The attack on Tuesday, which two human rights and medical groups say bears the signs of illegal chemical weapons use, puts the Trump administration in a bind. If chemicals were used by Assad's military, it would mean Syria violated a deal to destroy such weapons, an accord brokered by the Obama administration and Russia after an August 2013 sarin gas attack killed more than 1000 people in a Damascus suburb.

As a private citizen at the time, Mr Trump slammed proponents of greater involvement in Syria following that attack, saying the US should "stay away" from the crisis.