Iraq said on Thursday that it believed militants loyal to al Qaeda were crossing from Iraq into Syria to carry out attacks.
"We have solid information and intelligence that members of al Qaeda terrorist networks have gone in the other direction, to Syria, to help, to liaise, to carry out terrorist attacks," Iraqi Foreign Minister Hoshiyar Zebari told a news conference in Baghdad.
Syria says that a 16-month-old uprising against President Bashar al-Assad is not a popular revolt but a "terrorist" conspiracy funded and directed from abroad, not least by the wealthy Gulf monarchies of Saudi Arabia and Qatar.
It also says much of the violence in Syria bears the fingerprints of al Qaeda and its Sunni Muslim Islamist ideology.
UN Secretary-General Ban Ki-moon has said he believes al Qaeda was responsible for two suicide car bombs in Damascus in May that killed at least 55 people.