Guerrilla fighters trained by the West began moving towards Damascus in mid-August, French newspaper Le Figaro reported on Thursday.

Le Figaro reported that this is the reason behind the Assad regime's alleged chemical weapons attack in Damascus on Wednesday morning, as UN inspectors were allowed into the country to investigate allegations of WMD use.

The rebels were trained for several months in a training camp on the Jordanian-Syrian border by CIA operatives, as well as Jordanian and Israeli commandos, the paper said.

The first group of 300 handpicked Free Syrian Army soldiers crossed the border on August 17 into the Deraa region, and a second group was deployed on August 19, the paper reported.

The paper quoted a researcher at the French Institute for Strategic Analysis as saying the trained rebels group was passing through Ghouta, on their way to Damascus.

In June, the Los Angeles Times reported that CIA operatives and American special operations units have been training Free Syrian Army soldiers with anti-tank and anti-aircraft weapons since late 2012.

The newspaper reported that the training took place at covert bases in Jordan and Turkey.

So far, the Obama administration has been hesitant to sanction large-scale military aid to the rebels for fear that the arms could end up in the hands of radical Islamists currently fighting in the Assad regime.

Washington has been urged by lawmakers at home and critics abroad to increase involvement in the Syrian conflict, which has claimed the lives of tens of thousands in the last two years.

The United States has left about 700 combat-equipped troops in Jordan after a training exercise there, at the request of the Jordanian government, US President Barack Obama said on Friday.

"This detachment that participated in the exercise and remained in Jordan includes Patriot missile systems, fighter aircraft, and related support, command, control, and communications personnel and systems," Obama said.

A team of United Nations chemical weapons experts arrived in Damascus on Sunday to investigate the possible use of chemical weapons in Syria's civil war.

President Bashar Assad's government and the rebels fighting him have accused each other of using chemical weapons, a step which the United States had said would cross a "red line" in a conflict which has killed 100,000 people.

The UN team, including weapons experts from the Organization for the Prohibition of Chemical Weapons, will try to establish only whether chemical weapons including sarin and other toxic nerve agents were used, not who used them.