Trump blames Obama's 'weakness' for Syria chemical attack

Donald Trump blamed former President Barack Obama’s “weakness” for Tuesday’s suspected chemical weapons attack on rebel-held northwestern Syria, while his secretary of state said Russia and Iran bore moral responsibility for the deaths.

In a series of strongly worded statements, Trump's administration sought to convey a forceful response to the attack on Khan Sheikhun in Idlib province.

Trump said the attack against innocent people must not be "ignored by the civilised world."

"These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution," Trump said, in a reference to Obama's failure to strike in 2013 after saying a chemical attack by Assad would cross a US red line.

However at the time, Trump had called on Obama not to attack Syria, despite the horrific images showing the deaths of hundreds of Syrian citizens as a result of chemical attacks by the Assad regime.

Trump left it to his top diplomat, Secretary of State Rex Tillerson, to assign at least some blame to Russia and Iran, Assad's most powerful allies.

Tillerson called on both countries to use their influence over Assad to prevent future chemical weapons attacks.

He noted Russia's and Iran's roles in helping broker a ceasefire through diplomatic talks that have occurred in the Kazakh capital of Astana.

"As the self-proclaimed guarantors to the ceasefire negotiated in Astana, Russia and Iran also bear great moral responsibility for these deaths," Tillerson said.

Both Trump and Tillerson referred in written statements to a chemical weapons attack, rather than a suspected attack, suggesting the US has reached some degree of confidence about what took place in Idlib.

At the White House earlier Tuesday, spokesman Sean Spicer said the White House had received a number of phone calls from European allies questioning how it would address the problem, pressing Trump's "America First" administration to take a bold position on this civil war that has killed hundreds of thousands and prompted the worst refugee crisis since World War II.

"I'm not ready to talk about our next step but we'll talk about that soon," Spicer said.

The attack Tuesday is believed to have killed dozens of people, with activists on the ground describing the attack as among the worst in the country's six-year civil war.

Obama gave the Assad government an ultimatum that the use of chemical weapons in any circumstance would result in consequences.

But those consequences never materialised, especially with the landscape of the conflict growing more complicated by the rise of radical groups like al-Qaeda and the Islamic State group.

The Syrian conflict began when the Baath regime, in power since 1963 and led by President Bashar al-Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fueled by mass defections from the Syrian army.

According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.

Agencies contributed to this report.