"Strikes by US, France and UK make it clear that Syrian regime together with Russia and Iran cannot continue this human tragedy, at least not without cost. The EU will stand with our allies on the side of justice," Tusk said in a Twitter message.

EU Commission chief Jean-Claude Juncker said this was not the first time that Damascus had used chemical weapons against civilians "but it must be the last.

"The international community has the responsibility to identify and hold accountable those responsible of any attack with chemical weapons," Juncker said in a statement.

"As it enters its 8th year of conflict, Syria desperately needs a lasting ceasefire respected by all parties that paves the way for achieving a negotiated political solution through the United Nations-led Geneva process, to bring peace to the country once and for all."