Seoul - A "very serious" US response to an alleged chemical weapons attack by the Syrian government does not require UN corroboration that the incident took place, a senior Republican senator said on Monday.
Speaking as UN inspectors headed to the site near Damascus where the opposition says more than 1 300 people died last week, Senator John McCain said he was "totally convinced" President Bashar Assad's regime was behind the "atrocity".
McCain, who has long argued for military aid to the Syrian opposition to tip the balance in the conflict, said he was sceptical that the UN team would be able to complete a thorough probe.
"It's hard for me to believe that Bashar Assad is going to allow the UN to investigate something he has done that will be harmful to him," the former presidential nominee told reporters in the South Korean capital Seoul.
"But we don't need UN corroboration," McCain said, urging President Barack Obama to respond swiftly and effectively.
"If the US stands by and does not take very serious action - not just launching some cruise missiles - then again our credibility in the world is diminished even more... if there's any left," he said.
Obama had said a year ago that the use of chemical weapons by Assad's forces was a "red line" that could trigger Western intervention.
McCain said the United States should neutralise the Syrian government's air capability and establish a safe zone under its protection.
He also rejected arguments by some in the US administration that arming the Syrian rebels might result in weapons falling into the hands of Islamist groups.
"We can get the weapons to the right people. That is not a difficult task," he said.
McCain was in Seoul, together with Democratic Senator Sheldon Whitehouse, for a visit to discuss the human rights situation in North Korea.