European Union foreign policy chief Catherine Ashton has called the Syrian regime's use of chemical weapons a "war crime", and called for a "clear and strong response", but urged Washington to await the UN weapons inspectors' report into the attack.
Speaking in the Lithuanian capital, Vilnius, on Saturday, Ashton called the chemical attack a "blatant violation of international law, a war crime and a crime against humanity".
Ashton said that all 28 EU governments agreed that the evidence pointed to the Assad regime being behind the attacks in Ghouta and other rebel-held areas of Damascus on 21 August.
EU members were "unanimous in condemning in the strongest terms this horrific attack", she said.
She said the Syrian government was "the only one that possesses chemical weapons agents and the means of their delivery in a sufficient quantity", adding that the world "cannot remain idle".
However, she stopped short of calling for military action against the regime, as advocated by US President Barack Obama and his French counterpart, Francois Hollande.
US Secretary of State John Kerry, in Vilnius to lobby for support in launching air strikes, described Ashton's words as a "strong statement about the need for accountability".
President Obama has asked the US Congress to approve the use of force, with a final Senate vote expected in a week.
Ashton said EU ministers welcomed Hollande's commitment to wait for the UN inspectors' report before deciding to act.
"The EU underscores... the need to move forward with... addressing the Syrian crisis through the UN process," she said.
The US says that 1,429 people were killed in the 21 August attack. Aid agencies put the toll at 355 deaths, with thousands wounded.
In Damascus, heavy government shelling of rebel positions left 16 dead on Saturday, activists said.
Mortar and artillery fire in the Moldokhiya area south of Damascus killed 14 rebels, said the Syrian Observatory for Human Rights.
Fighters from the al-Nusra Front, an al-Qaida linked splinter group at the vanguard of rebel forces, advanced on the Christian village of Maaloula, northeast of the capital, amid heavy fighting.