U.S. President Barack Obama's administration has stepped up efforts to muster domestic support for military action against Syria.
Mr. Obama, in his weekly radio address, said failing to respond to what he called Syria's "outrageous attack" would increase a risk of further chemical weapons attacks.
In the address, which was broadcast on Saturday, he also said the Syrian government's alleged use of chemical weapons was a "direct attack on human dignity" and a threat to national security.
As the president seeks to strengthen domestic support for action against Syria, U.S. Secretary of State John Kerry is pressing European foreign ministers to back the effort.
Kerry held informal discussions with his fellow diplomats Saturday in Vilnius, the capital of Lithuania, which currently holds the European Union's rotating chair.
After meeting with Kerry, EU foreign policy chief Catherine Ashton read a statement for the group that called for a "clear and strong response" to the attack in Syria but stopped short of specifying military action.
Ashton also said the EU wants Syria's crisis to be addressed by the United Nations. She said any further action against Damascus should be delayed until a U.N. chemical weapons team presents its findings.
Kerry responded, saying the U.S. was grateful for the support. He said the EU had issued a "strong statement about the need for accountability."
Also, Germany said it would sign a G20 statement calling for a "strong response" to the attack. The U.S. and 10 other countries signed the statement Friday, during a G20 meeting in Russia.
At the G20 summit, Mr. Obama held bilateral meetings on Syria with leaders of the world's major economies. He received support from the leaders of France, Turkey and other nations, but Russian President Vladimir Putin remained adamantly opposed to any attack on the government of Syrian President Bashar al-Assad.
Mr. Obama takes his case for targeted military strikes on Damascus directly to the American people in a televised speech Tuesday.
The U.S. Congress is expected to vote on possible U.S. action against Syria in coming weeks. A key Senate panel voted on Wednesday in favor of action.
U.S. officials say they have evidence that more than 1,400 people were killed by poison gas in an attack on August 21 in areas on the fringe of Damascus populated by supporters of the opposition.
In Syria Saturday, opposition activists say at least 16 people were killed in heavy shelling near Damascus.