The United States on Tuesday added Syria to a blacklist of countries failing to abide by conventions barring human trafficking and slavery, a move which leads to sanctions and cuts in US aid.

"The government of Syria does not fully comply with the minimum standards for the elimination of trafficking and is not making significant efforts to do so," the 2012 Trafficking in Persons report said.

It voiced particular concerns about Filipina maids being sent to Syria via Dubai who were vulnerable to "conditions of forced labor" and to allegations reported by the United Nations that the Syrian opposition was using child soldiers.

US officials had not been able to fully assess the situation on the ground due to fierce fighting in the 15-month conflict, as the opposition seeks to oust President Bashar al-Assad, the report said.