Syria chemical weapons: 'A red line that has been crossed'

The United States has delivered a letter to UN Secretary-General Ban Ki-moon providing its latest assessment of chemical weapons use in Syria.

National security adviser Susan Rice said that the US had assessed with "high confidence" the use of chemical weapons in Syria, including the nerve agent sarin.

She said that President Barack Obama has made it clear that the use or transfer of chemical weapons was "a red line which has been crossed".

Syria has dismissed the claims as "a caravan of lies", after the US said it would give the rebels "direct military aid".