Syria crisis: 'Iraqi Shia fighters' join regime in battle
20 May 2013 Last updated at 10:38 BST
The BBC has seen evidence that the Syrian regime is getting support from Shia fighters crossing into the country from Iraq.
In an interview with the BBC's Aleem Maqbool, one man claimed to have recruited and prepared some of the "thousands" of Iraqi Shia fighters travelling across the border.
Meanwhile the tension is said to be contributing to increasing violence in Iraq, with fears the country could return to the worst of the sectarian conflict seen in 2006 and 2007.
Aleem Maqbool reports from Baghdad.