A United Nations team of weapons inspectors has returned to Syria to continue their investigation into allegations of chemical weapons use during the country's civil war.

Lebanese airport officials told the Associated Press that the team, led by Swedish expert Ake Sellstrom, arrived in Beirut Wednesday morning on its way back to Syria. Reuters reported that at least eight members of the team were seen arriving at a Damascus hotel in a convoy of five United Nations cars at around midday Wednesday.

A statement by the U.N. on Tuesday said the inspectors will use their new visit to gather evidence from the alleged chemical weapons attack on March 19 on the village of Khan al Assal outside the city of Aleppo, which was captured by the rebels in July.

All officials spoke on condition of anonymity because they were not authorized to talk to the media.

A report following the inspectors' previous trip said nerve agent sarin was used in an August attack near Damascus.

The U.S. and its allies say President Bashar Assad's regime was behind the attack. Washington says the attack killed 1,400 people. Activist groups gave significantly lower death tolls, but still in the hundreds.

Meanwhile, several Syrian rebel groups, including a powerful Al Qaeda-linked faction, say they reject the authority of the Western-backed opposition coalition in exile.

In a joint statement released on Wednesday, 13 rebel groups led by the Al Qaeda-linked Nusra Front slammed the Turkey-based Syrian National Coalition, saying it no longer represents their interests.

The statement also called on all those fighting to topple President Bashar Assad's regime to unite under a "clear Islamic framework" -- an apparent reference to the Al Qaeda faction's aspirations to create an Islamic state in Syria.

The Associated Press contributed to this report.