U.S. President Donald Trump makes a statement on health care while standing with 'victims of Obamacare' at The White House on July 24, 2017 in Washington, DC.

President Donald Trump harshly condemned on Sunday a suspected chemical attack in Syria, vowing that the country's president, Bashar al-Assad, and Russia would pay a "big price" for their role in the crisis.

In what amounted to his strongest critique yet of Russia, Trump singled out Russian President Vladimir Putin for supporting Assad. Since 2011, the Assad government has been locked in a deadly and protracted conflict with resistance fighters, and elements of ISIS, and has been backed by Russian forces.

As international condemnation mounted, both Syria and Russia dismissed reports of the chemical attack in the city of Douma, which is said to have taken place in a rebel-controlled area near Damascus.

Yet on Twitter, Trump excoriated both Assad and Putin for their roles in the ongoing violence. Calling Assad an "animal," the president also lashed out at Putin for supporting the embattled leader. Trump's remarks were significant, given that Putin has been one of the few world leaders the president has refrained from criticizing publicly.

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Trump also took aim at his predecessor, Barack Obama, for failing to confront Syria back in 2012, when the country was first suspected of gassing civilians. At that time, Obama famously stated Assad would cross a "red line" if he attacked rebel forces with chemical arms, but declined to enforce it.

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A medical relief organisation and a rescue service said Sunday that 49 people were killed by a chemical attack on a rebel-held town, but the BBC put the death toll much higher. No independent verification of the dead has taken place yet.

Trump's condemnation comes a year after he ordered a missile strike against Syria for a similar attack. In April of 2017, U.S. forces rained down dozens of Tomahawk missiles on a Syrian base, after dozens of civilians were killed in a chemical attack.

The president's stronger tone may reflect the influence of John Bolton, his newly appointed national security advisor and an avowed defense hawk who has openly discussed taking military action against Syria.

Trump has also blamed Iran for its backing of the Assad regime, and Bolton is an opponent of the Obama-era nuclear deal negotiated with Tehran.

The Syria Project at the Foundation for Defense of Democracies notes that, "In the long run, Assad's dependence on Iran and its proxy militias – which have declared their intent to permanently remain in the country – will serve to erode Syrian sovereignty, perhaps transforming Damascus into another puppet of Tehran."

Last week, Trump floated the idea of extricating U.S. forces from the conflict, but his advisors have downplayed those remarks.