The United States is stepping up support for the Syrian rebels but, despite the failure of UN envoy Kofi Annan's mission, is sticking to its refusal to pour arms into a complex conflict.

Annan, the UN and the Arab League's pointman on Syria, resigned yesterday leaving the diplomatic track in shreds and spurring calls for action, but the US administration again said it was not considering military intervention.

The 17-month uprising against Assad has already cost more than 20,000 lives, many of them civilian, and raised fears that the embattled Syrian leader could unleash his arsenal of chemical weapons.