* UN has received 13 reports of chemical weapons use
* Investigators due to visit three sites of possible attacks (Adds detail throughout)
By Michelle Nichols
UNITED NATIONS, Aug 13 (Reuters) - U.N. experts are ready to visit Syria to investigate claims of chemical weapons use during the country's two-year civil war, but a deal has not yet been reached with the Syrian government on safety assurances, the United Nations said on Tuesday.
"The devil is in the details and the details are being worked out," U.N. spokesman Eduardo del Buey told reporters.
It has been nearly two weeks since the United Nations said the Syrian government had agreed to let the U.N. team of experts travel to three sites where chemical weapons are reported to have been used. One, Khan al-Assal in Aleppo, is where the Syrian government says rebels used chemical weapons in March.
The other two locations to be visited have not yet been identified. The United Nations said it has received 13 reports of possible chemical weapons use - one from Syria's government and the rest mainly from Britain, France and the United States.
The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied it.
U.N. Secretary-General Ban Ki-moon appointed Swedish scientist Ake Sellstrom in March to lead a U.N. inquiry into the claims, but diplomatic wrangling and concerns over safety have prevented Sellstrom and his team of experts from entering Syria.
The U.N. inquiry will only try to establish whether chemical weapons were used, not who used them. Sellstrom's team is made up of experts from the Organization for the Prohibition of Chemical Weapons and the World Health Organization.
"Over the weekend, the investigation team ... completed all necessary logistical arrangements for its visit to Syria," the United Nations said in a statement on Tuesday.
But U.N. High Representative for Disarmament Affairs, Angela Kane, was still negotiating with the Syrian government on "the modalities essential for cooperation to ensure the proper, safe and efficient conduct of the mission," the world body said.
"Once the government of Syria confirms its acceptance of the modalities, the mission will depart without delay," the United Nations said.
Rebels seized Khan al-Assal from Syrian President Bashar al-Assad's forces last month. The opposition Syrian National Coalition has written to Ban saying they were ready to cooperate with the chemical weapons inquiry and "welcome U.N. investigators into all territories under our control."
The United Nations has been demanding unfettered access in Syria to conduct the chemical weapons investigation.
Syria is one of seven countries that has not joined the 1997 convention banning chemical weapons. Western countries believe it has stockpiles of undeclared mustard gas, sarin and VX nerve agents.
The United Nations says more than 100,000 people have been killed in the Syrian conflict.
More than 1.9 million Syrians have fled the country - two-thirds of those since the start of the year - and more than 4.2 million people have been internally displaced, the United Nations has said. Most of those in need are women and children. (Reporting by Michelle Nichols; editing by Jackie Frank and Cynthia Osterman)