If the US launches military action against Syria over the suspected poison gas attack, it could not only escalate troubles there but also in the wider region. The warning, as Donald Trump's moves raise fears of direct conflict between Washington and Moscow.
Drawing a 'thick red line'
For one Brussels commentator, it's not a case of if, but when there'll be a military strike.
"It has to happen because it's pretty clear that when it did not happen in 2013, this has basically led to a series of unfolding events where the Assad regime has kept on using chemical weapons against the population," said Fabrice Pothier, Chief Strategy Officer at Rasmussen Global.
"So I think it's pretty clear for Washington and Paris that now we have to somehow draw that thick red line."
Dozens of people died in Douma, and hundreds were injured in the alleged attack, according to the World Health Organization.
It's unclear what kind of military action - and its scale - would be used against Syria - if Trump does decide to go down that route.
Experts say what targets are hit and what damage is caused will determine the repercussions.
"If the Americans hit the Syrian army, and only the Syrian army, then I think the Syrian regime may take revenge by hitting civilians in rebel-held areas in Syria. And that would be terrible for the civilians but that would still be a contained conflict," said Joost Hiltermann from the Brussels-based International Crisis Group.
"However, if the Russians get really upset about what is hit, and maybe Russian troops are hit or airplanes, and the same for the Iranians, then you could see an escalation not only in Syria but elsewhere in the region."
EU to discuss Syria
Euronews' Damon Embling reported: "Here in Brussels, the European Commission issued a statement of condemnation, after what's said to have happened in Douma. It's not so far commented on the subsequent threat of military action, but a source has told me they're closely monitoring the situation with Syria - and it's due to come up for discussion at a Foreign Affairs Council meeting on Monday."