The joint statement signed by two dozen countries calls to hold accountable those who are responsible for the use of chemical weapons and a strong international response to it.
The initial 11 signatories to the joint statement issued on the sidelines of last week's G20 conference are Australia, Canada, France, Italy, Japan, South Korea, Saudi Arabia, Spain, Turkey, Britain and the US.
"We welcome additional countries expressing their support for this statement and our continued efforts to hold the Assad regime accountable and enforce the international norm against the use of chemical weapons," the White House said in a statement.
"We call for a strong international response to this grave violation of the world's rules and conscience that will send a clear message that this kind of atrocity can never be repeated. Those who perpetrated these crimes must be held accountable," the joint statement said.
"The world cannot wait for endless failed processes that can only lead to increased suffering in Syria and regional instability. We support efforts undertaken by the United States and other countries to reinforce the prohibition on the use of chemical weapons," it said.
The US has alleged that the nerve agent sarin was used by the Assad regime on August 21 that left at least 1,429 people killed, a charge denied by the Syrian government.