(Warning: the map below contains distressing images and links to videos which contain disturbing content)
Click on the map above to see the sites where, it is claimed by rebels, the Syrian government has used chemical weapons.
Channel 4 News has viewed thousands of videos posted by rebel groups on Youtube, including some videos that purport to show evidence of chemical weapons use.
According to this uncorroborated evidence, which covers alleged attacks from April 2012 until April 2013, the Syrian government has used chemical weapons in cities such as Damascus, Homs and Aleppo, and also in more rural locations.
The videos typically show men, women and children alleged to have been caught up in a chemical gas attack. They include footage of children covered with chemical burns and victims frothing at the mouth as they receive treatment. Many are too distressing to show in full.
The prime minister said on Friday that there was “limited but growing evidence” of chemical weapons use by the Syrian government. He said this was “extremely serious” and “a war crime”, but added that it was unlikely British troops would go into Syria.
On Thursday the White House said it believed chemical weapons had been used on a small scale, but insisted that US President Barack Obama would need to have definitive evidence of their use before the country took action.
On Friday the US government said it was continuing to look at the evidence, but would not set a timetable for corroborating reports.
White House spokesman Jay Carney, speaking about potential US responses, said of Barack Obama: “He retains all options to respond to that, all options.
“Often the discussion, when people mention all options are on the table, everyone just talks about military force. It’s important to remember that there are options available to a commander in chief in a situation like this that include but are not exclusive to that option.”