US secretary of State Hillary Clinton on Thursday, laid out arguments against armed intervention in Syria despite last week's massacre in the town of Houla.

Speaking to Danish students, Clinton got tough questions on what might motivate the US and others to take military action in Syria, where President Bashar al-Assad is battling a 14-month-old anti-government uprising.

Friday's massacre of more than 100 civilians, many of them children, in Houla has triggered calls for the West to take more robust action in Syria, despite Russian and Chinese opposition.

However, Clinton rehearsed US arguments against armed intervention for now in contrast with Libya, where Western-led air strikes last year helped end Muammar Gaddafi's rule.

Clinton said Syria had a more diverse society with greater ethnic divisions, no unified opposition, stronger air defences and a more capable military than Libya's.

Above all, she stressed there was no international support because of Russian and Chinese opposition at the UN Security Council, where they have twice vetoed resolutions on Syria.

Speaking later at a news conference with the Danish foreign minister, Clinton said she would try to change Russia's stance.

"The Russians keep telling us they want to do everything they can to avoid a civil war because they believe that the violence would be catastrophic," she said.

She told the students that Syria's population density increased the odds of civilian casualties in any armed action.