* U.N. experts to initially stay in Syria up to two weeks
* U.N. will investigate if chemicals used, not by whom (Adds details throughout)
By Michelle Nichols
UNITED NATIONS, Aug 14 (Reuters) - U.N. experts will travel to Syria imminently to investigate claims of chemical weapons use during that country's civil war after the United Nations and the Syrian government agreed on details of the trip, U.N. Secretary-General Ban Ki-moon said on Wednesday.
The United Nations said two weeks ago that Syrian President Bashar al-Assad's government had agreed to let the inspectors, led by Swedish scientist Ake Sellstrom, travel to three sites where chemical weapons were reported to have been used.
Ban's press office said in a statement on Wednesday that the Syrian government had now "formally accepted the modalities essential for cooperation to ensure the proper, safe and efficient conduct of the mission."
"The departure of the team is now imminent," it said. "As agreed with the government of Syria, the team will remain in the country to conduct its activities, including on-site visits, for a period of up to 14 days, extendable upon mutual consent."
The United Nations said Sellstrom's team had completed their trip preparations in The Hague last weekend.
One site to be visited by the U.N. experts is Khan al-Assal in Aleppo, where the Syrian government says rebels used chemical weapons in March. The other two locations have not yet been identified.
Rebels seized Khan al-Assal from Assad's forces last month. The opposition Syrian National Coalition has told Ban it would cooperate with the chemical weapons inquiry and "welcome U.N. investigators into all territories under our control."
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.
CHEMICAL WEAPONS A U.S. 'RED LINE'
The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied it. The U.N. inquiry will try to establish only whether chemical weapons were used, not who used them.
The United States concluded in June that Assad's forces used chemical weapons against rebel fighters. President Barack Obama said last year that any attempt to deploy or use chemical or biological weapons in Syria would cross a "red line."
U.S. congressional panels last month agreed to a White House plan to provide arms to the Syrian rebels.
Ban appointed Sellstrom in March to lead a U.N. inquiry into the claims of chemical weapons use, but diplomatic wrangling and concerns over safety have prevented the team of experts from entering Syria.
"The Secretary-General believes that an effective mechanism to investigate allegations of the use of chemical weapons can serve as an important deterrent against their employment," Ban's press office said.
The United Nations has been demanding unfettered access in Syria to conduct the investigation. Sellstrom's team is made up of experts from the Organization for the Prohibition of Chemical Weapons and the World Health Organization.
Syria is one of seven countries that have 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 since 2011.
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. (Editing by Will Dunham and Mohammad Zargham)