UN chief Ban Ki-moon insisted Monday that UN inspectors be granted unrestricted access to Syrian sites where chemical weapons were allegedly used, as the experts began their mission.

Inspectors, who are expected to visit three sites including Khan al-Assal near Aleppo, are due to be in Syria for 14 days, with the possibility for extension of the mission if there is mutual consent.

"In order to credibly establish the facts, the mission must have full access to the sites of the alleged incidents," Ban told reporters.

He said investigators needed to be able to collect samples as well as conduct interviews and examine witnesses, victims, and attending medical personnel. The team will also conduct autopsies.

The United Nations says it has been given reports on 13 suspected chemical weapons attacks during the war.

The United States and other Western nations believe chemical arms have been used in limited quantities by forces loyal to Syrian President Bashar al-Assad.

Ban emphasized that the mission would not determine who had used chemical weapons but only whether chemical weapons were used. He said he hoped findings would help deter their use in the future.

"If confirmed, the use of chemical weapons by any side under any circumstances must be held accountable and would constitute an international crime," Ban affirmed.

A chemical weapons attack was reported in March in Khan al-Assal, which is located near the key city of Aleppo.

The government in Damascus -- which agreed to allow access to the UN team last month -- has accused rebel forces of using chemical arms in Khan al-Assal. The rebels blame the army for the attack.

The UN has not confirmed the location of the other two sites to be inspected.

Ban said the "government and all other entities within Syria" must ensure the safety of the UN team, which is composed of a dozen inspectors and is led by Swedish expert Ake Sellstrom.

At the UN Security Council, during a debate on protecting civilians in armed conflict, UN rights chief Navi Pillay and UN humanitarian chief Valerie Amos renewed their call for more humanitarian access in Syria.

"Insecurity, coupled with bureaucratic constraints and limitations on the number of NGOs allowed to operate in Syria, continue to prevent aid reaching all those in need," Amos said via video conference.

Pillay reiterated her call for the UN Security Council to refer the situation in Syria to the International Criminal Court in The Hague, which the split 15-member council has so far been unable to do.